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I am working on this question:

An assembly operation for a computer circuit board consists of four operations that can be performed in any order.

a) In how many ways can the assembly operation be performed?

b) One of the operations involves soldering wire to a microchip. If all possible assembly orderings are equally likely, what is the probability that the soldering comes first or second?

In part a, I just use $4! = 24$

In part b, I can't get it, I'm a little confused about "comes first or second". Should I get the individual probabilities and just add them?

My answer in part b is $\frac{6}{64} + \frac{6}{64}$. Is that correct?

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    Its rather unfortunate that you ask us 5 questions in a row in an hour or so. Please don't try to get your home work issues sorted out here.2012-01-22

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Yes, you should get the individual probabilities and add them.

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    I always like to see what answers get through the see-if-I'm-a-human-contributing-a-valuable-answer screen. This one (a generic agreement) did, although some short answers (but that actually have had content) of mine in the past haven't. How funny.2012-01-22
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    my answer in letter b is 6/64 + 6/64. is that correct?2012-01-22
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    Let's see - how many total possibilities are there? There are $4! = 24$ total combinations, each equally likely. How many have soldering first? Choose soldering, then there are $3! = 6$. So the probability that soldering comes first is $6 / 24$. I don't think that's how you did it, right?2012-01-22
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    and since each event are equal, the second event (which is to be the second) is 6/24 also?2012-01-22
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    @user: That's a good question. Are they really?2012-01-22
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    yes since. it would be like 3 x 1 x 2 x 1 = 62012-01-22
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    An interesting symmetry principle is at work here. The labelling of the positions in the ordering of the operations as "first", "second" etc. is completely arbitrary, so the probability of some operation being first is the same as the probability of the same operation being second.2012-08-03
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    Some people would try to calculate the probability of soldering being second by adding up the cases according to which comes first, then soldering second. But that is completely unnecessary.2012-08-03
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Answer to part b) of your question is 0.5