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original post

the examples here are, the most important word -- fundamentally -- the same.

  • example1: the most abstract way to present this example.

    Why equivalent % increase of A in event1 and % decrease of B in event2 is not the same end result in these two cases/events? % is percent chance of curing cancer. that logically means A = [total cancer potency], and B = [whatever the trade is for increasing the chance of curing cancer] ..and the "naming" of A and B is defined by choice

  • example2: concrete

    1 cupcake = 1 karma

    case/event 1

    • 66% increase of 100 cupcakes = .66 * 100 cupcakes = 66 increase, so total 166 cupcakes
      so the outcome is 166/100 = 1.66 cupcakes per karma

    case/event 2

    • 66% decrease of 100 karma = .66 * 100 karma = 66 decrease, so 34 total karma
      so the outcome is 100/34 = 2.9 cupcakes per karma

so to me and everyone, 66% increase and 66% decrease seems like they would lead to the same end result/outcome, BUT as shown by math calculation, the resulting outcome of each event is not the same. you are actually getting more cupcakes if you decrease the karma than you would if you increase the amount of cupcakes, which doesnt make any sense at all. you think to yourself how could this have happen? how can this be? the world doesnt make any sense. see talk for more.

looking for a non-math-calculation explanation, (cogintive, neuroscientific, lingiustical, philosophical, whatever is useful, etc.) using plain english, not math language since i was never taught it properly or clearly

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a related question after thinking about the talk, with some deleted comments, below -- isnt it true that equal % increase of A is going to have the same end result as an equal % decrease in A if they have the same initial number?

  • if true, ok, so this would support the thinking in the original post
  • if false, i need to correct my assumptions, and yet, nobody has yet to explain clearly and helpfully why this is

i think this is true because they are equal % from the same initial number.

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    umm.. if i use 50% to think about this, it really didnt help, 50% increase means you get 50% (half) more, 50% decrease means you get 50% (half) less2012-10-01
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    Ity should have helped. Start with 100 dollars. Fifty percent more is $150$. Fifty percent less than that is $75$.2012-10-01
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    In the scenario when you go half up and then go half down, going half down takes you down by _more_ than going half up takes you up.2012-10-01
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    "if you decreased first, you would be increasing half by half, which would mean adding 25% to 50%, still resulting in 75%." -- ok, but why? non-math answer -- "going half down takes you down by more than going half up takes you up" -- why?2012-10-01
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    "50%" is not a fixed amount, like 100 dollars. "50%" has to be 50% *of* something. When you go up 50%, and then down 50%, you are not taking 50% of the same thing each time, so you will not end up at the same place you started.2012-10-01
  • 0
    i already know that but this is not an example where a percent increases, THEN it decreases, it's an example about two cases where it (A) increase or (B) decrease ONE time, and the % of that increase and decrease is the SAME -- so "quantity of cupcakes" and "cost of cupcakes" can be starting at the same OF, the same SOMETHING2012-10-01
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    edit: "i already know that but this is not an example where a percent increases for A, THEN it decreases ALSO for A"2012-10-01
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    @high if they have the same starting points, then they get the same amounts. A $50\%$ increase from $100$ and a $50\%$ decrease from $100$ both give a difference of $50\$$. If that wasn't your complaint, what are you referring to?2012-10-01
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    This sounds like a linguistic complaint then. That you think $50\%$ increase and $50\%$ decrease should do the same thing?2012-10-01
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    yes, and everyone thinks this though (UNTIL they get into -- learn -- a way of thinking about math that doesnt make any sense to everyone because that's not how they think -- ppl dont think it these math-created rules), this question was based on something about how ppl dont understand math -- if this made sense to me, i would've understood by now, but i dont because nobody has yet provided an english explanation, everyone is going off of math lang because that is how they learned it -- but the point is, it fundamentally doesnt seen to make any sense2012-10-01
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    ppl arent born thinking 50% increase and 50% decrease from the same starting points should NOT do the same thing -- everyone naturally thinks it does -- UNTIL math lang proves that mistaken -- but there's no clear explanation. i wont be able to teach this to a 7 yr old, 21 or 55 yr old girl, because i was never taught it -- and neither can you -- and those FEW who are, well, where are they? and what educational institutions they go to? likely a progressive, very new one in the gloabl society i havent heard about2012-10-01
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    right now, the only reason for "50% increase and 50% decrease, from the same starting points, should NOT do the same thing" is a math calculation. i was looking for a clear, concrete one that anyone would understand. i believe there is that, but ppl dont know how to explain it.2012-10-01
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    the highest levels of mathematicians from history are probably are able to explain this clearly. they i think are able to bridge the divide between math lang and human lang. they understood that math does reflect the real world (i dont understand this, that's why i separated the "math world" from the "real world") -- im not able to bridge this divide -- and neither was anyone here so far2012-10-01
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    see related reasoning for "ppl dont know how to teach" http://math.stackexchange.com/questions/204154/better-way-to-find-of-a-number-y-equal-to-another-number-x2012-10-01
  • 1
    If you don't think that "increase" and "decrease" mean the same thing by themselves, why would "50% increase" and "50% decrease" mean the same thing? I don't even know *why* you're so confused. There is no "math world" vs "real world" going on, there is "you don't understand what this particular English statement means". This isn't a math question, it's a language question.2012-10-01
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    no, you have an incorrect assumption/understanding of what i said -- they do mean the same in opposite ways. one is increasing by the SAME percent, and the other is decreasing by the SAME percent. this is why the end result is expected to be the way, but it is not for the A and B examples2012-10-01
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    since the statement "There is no "math world" vs "real world" is based off of (devires from) the incorrect assumption/understanidng/premise cleared up in my last comment, that statement is wrong and was already explained in this talk, and expanded on further, very quckly, with the mathematicians from history comments2012-10-01
  • 0
    edit: "this is why the end result is expected to have the same outcome"2012-10-01
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    I'm going to take a single stab at this. In example 2 in your post, you want to relate a 66% increase in the number of cupcakes to a 66% decrease in the cost of cupcakes. In general, there is no reason why two any quantities should be related. Here, in this particular example, there is no reason to assume an increase in the number of cupcakes would result in a decrease in cost. Consider a grocery store receiving new items to put on the shelves. The price doesn't magically decrease. Are you thinking of supply and demand?2012-10-01
  • 0
    Okay, so say two people have \$100 each. One increases theirs by 50%, and the other decreases theirs by 50%. You think they should both end up with the same amount of money? If I just said "one increases their $100, while the other decreases theirs" with no percentage, then you would recognize that increase and decrease are opposites, right? Why would that change just because we stuck a number next to it? You seem to be looking at the number and skipping over the rest of the sentence that describes what the number *means*.2012-10-01
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    "You think they should both end up with the same amount of money?" -- I think the "amount that is being added and deducted" from each case/event/person should end up to be the same, but this example is not accurate to the OP example. the OP example is "A increasing in Event1 and B decreasing in Event2." here, you have used the "A increasing in Event1, and A decreasing in Event2" example -- you had completely left out B2012-10-01
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    "should end up to be the same" -- and by "the same" i mean +50 is basically -50, just that they are opposites and inverses, but the 50 is the same, so fundamentally, the amount that gets added and deducted is "fundamentally the 'same'" -- EXAMPLE, like dark light is the SAME as bright light, they are both LIGHT, just one is dark, and one is bright, which can be understood as "inverses" or "opposites" -- this example used is what is called non-math lang, and it's far more easy for anyone to understand2012-10-01
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    because that is how we, naturally, from birth, conceptually think -- the object -- the dog, the cat, light, whatever, is the same. so canada and the usa is the SAME, they're both countries, JUST with diff governmental structure, diff location, diff amount of population, different social issues, diff other things and similar other things, BUT THEY ARE BOTH UNDER A GOVERNMENT, and meeting that condition, are thereby legally called a "country" that has human beings -- and that fundamentally makes them the same.2012-10-01

4 Answers 4

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The current question seems to be this. Sorry, but this is "math language" with some prose sprinkled in.

Say I have 100 cupcakes selling for 1 dollar each. If I increase the number of cupcakes by 66%, then I'll have 166 cupcakes. $$\frac{\text{# of cupcakes}}{\text{price}}=\frac{166}{100}=1.66$$

If I have 100 cupcakes selling for 1 dollar each, and I instead decrease the price by 66%, then I'll be selling each one for 34 cents each, and we get $$\frac{\text{# of cupcakes}}{\text{price}}=\frac{100}{34}=2.94$$

So the issue seems to be this: Increasing the denominator by some percentage and decreasing the denominator by the same percentage don't give the same result. So what gives? Maybe this is just me, but I find that math helps me make this intuitive. Look at multiplying by a percentage like this: adding 66% is multiplying by 1.66=(1+.66), and subtracting 66% is like multiplying by .34=(1-.66). So if our percentage is $x$ (66% here), we find that

$$\frac{a(1+x)}{b}\neq\frac{a}{b(1-x)}$$

plug some numbers in to see that this is the same situation. Now $a$ and $b$ cancel, so we find that this is just saying that $$1+x\neq\frac1{1-x}$$

Now comes the real question: Why might people expect something different to happen? I think we need to look at what people think is happening here, where intuition works perfectly, and see why they're applying it somewhere where it doesn't belong.

Let's look at a different situation. The one people might think the above is: scaling. If we scale the numerator up by $k$, then we get

$$\frac{ka}b$$

If we scale the denominator down by $k$, we get

$$\frac a{\frac 1 k b}=\frac {ka} b$$

So here they are the same! This is what you might think the above situation was. But it clearly isn't. So what's the difference?

The difference is that you were talking about adding and subtracting percentages. So when you say you took off 66%, this is the reverse of adding 66%. But fractions don't work with addition in this way:

$$\frac{5+2}{3}\neq\frac{5}{3-2}$$

If you phrased it in terms of multiplication, everything would work. The reverse of scaling up is the reverse of scaling down. So let's repeat, but doing that instead.

Say I have 100 cupcakes selling for 1 dollar each. If I scale the number of cupcakes up by 1.66, then I'll have 166 cupcakes. $$\frac{\text{# of cupcakes}}{\text{price}}=\frac{166}{100}=1.66$$

If I have 100 cupcakes selling for 1 dollar each, and I instead scale the price down by 1.66, then I'll be selling each one for 34 cents each, and we get $$\frac{\text{# of cupcakes}}{\text{price}}=\frac{100}{\frac{100}{1.66}}=\frac{100}{60.24}=1.66$$


So the best I can say is that some people think that adding/subtracting percentages is the same thing as multiplying/dividing by them, which it isn't. A 66% decrease doesn't undo a 66% increase. This is because percentages are all about scaling a number, so talking about adding and subtracting them in the first place is really just horribly obscuring what's really going on. This I think is a language issue. Look at the differences here:

"Let's say I have 100 people who have 100 houses between them. If I double the number of people, then there will be 2 people to every house. If I halve the number of houses, there will be two people to every house."

"Let's say I have 100 people who have 100 houses between them. If I add 100% of people, then there will be 2 people to every house. If I subtract 100% of houses, there will be 2 people to every house."

Some careful reading would reveal that the second situation is wrong, and that the two are not the same. Doubling is the same thing as adding 100%, but halving is not the same thing as subtracting 100%. Talking about adding/subtracting percentages is just awkward and obscures what's really going on, which is scaling.

  • 0
    Incidentally, for small percentages, "math language" makes it clearer that $\frac{1}{1-x}$ will be *close* to $1+x$. (They agree to first order near $x=0$.) For example, $0<\dfrac{1}{1-x}-(1+x) =\dfrac{x^2}{1-x}<2x^2$ when $02012-10-01
  • 0
    no, only if you already think/read in math lang, not when you dont understand it, like what was just written completely bewilders me and everyone else, so foreign2012-10-01
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    will read and think carefully about this on non-sleeping time -- thanks -- forewarning the answerer though, that even if i understand everything bypassing the translation gap, i dont think it'll be satisfactory because the question "why" specifically requests a clear non-math answer/example, but we'll see, i may be able to translate the math into real-world, practical, helpful understanding2012-10-01
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    If your'e working with numbers, you have to work with math. This is kind of like asking someone to speak French in Russian. There are some ideas that just don't translate properly (or at all) to other languages. Sometimes you just can't translate, and the only option is to gain some level of fluency/familiarity.2012-10-01
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    yes, yes i believe all or most can be translated somehow though (not precisely, but it can in a sufficient manner) -- clean-cut argumentation on http://math.stackexchange.com/questions/205304/solved-what-is-the-best-or-most-accurate-phrase-to-describe-this-set-of-vari --- if i eventually find out math really cant, then this means it is in such a seperate world apart from everythin else that this would have very serious implications, but i honestly dont expect this to be the case -- i believe most ppl just arent teaching it right2012-10-02
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    But you're not asking people to teach you math, you're asking people to give you English descriptions of math. If I explain that "scendere in pista" means "to take the floor" in Italian, then you aren't learning Italian (literally it's "to go down the trail") and you won't get anything useful out of it beyond the literal phrase I gave. If you want to *really* learn the concepts, you have to learn the language they're expressed in. Half the reason this is all so awkward and confusing is because you insist on seeing a 'translation' rather than the natural 'language' we're working in.2012-10-02
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Qualitative Approach

Consider it this way. Suppose you increase $A$ by $p$% giving $A^+$ and then decrease $A^+$ by $p$%. Because $A^+\gt A$, the $p$% decrease of $A^+$ will be greater than the $p$% increase of $A$, thus the end result will be less.

Likewise, suppose you decrease $A$ by $p$% giving $A^-$ and then increase $A^-$ by $p$%. Because $A^-\lt A$, the $p$% decrease of $A$ will be greater than the $p$% increase of $A^-$, thus the end result will again be less.

Quantitative Approach

$A(1-p)(1+p)=A(1-p^2)

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I know this post is old, but I ran across it and thought I would add some input.

You are looking at % as numbers, but they are not numbers, they are relationships between numbers.

You have 10 socks, give half of them away. You give away 5. Thats a decrease of 50%. But now you have 5 socks, and someone gives you 5, so you have 10. Thats an increase of 100%.

What you are thinking of is % increase/decrease of the same number is the same %. Mark a product that is 10 dollars up by 10% it's 11 dollars. Its on sale by 10% its 9 dollars.

But there is a fundamental difference between these two scenarios. The relationship of 10% of 10 will always be the same. The relationship is 1. The relationship of a % against different numbers will be different.

Example: I have an Aunt Debbie. Its my Mothers sister. Aunt Debbie is not an aunt to my mother. They are sisters. To my father, she is his sister-in-law. Shes the same person, but her relationship is different when applied to different people.

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you need to think about it this way. 10% increase of 100 = 110. 10% decrease of 110 --> 10% of 100 = 10 10% of 10 =1 10+1=11 110 - 11 = 99