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Find the third quartile class and frequency of the class from the given graph. How many students are there above the class? enter image description here My attempt : Here, $N=120$. Third quartile lies in $\frac {3N}{4}$item. $$=90 item$$ So, $Q_3 class = 20-30$.

I have previously asked this question at Graph-Third Quartle

But the actual answer given in my book is $30$ and the answer given by occasional user there is $20$. How is this difference? Which one is correct?

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    It seems like they're looking for the range of frequency rather than class, as that's about 30, and 20 if people forget to normalize to N=120. – 2017-01-28
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    @Vedvart1, I could clearly see three different questions here – 2017-01-28
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    1. Find the third quartile class. – 2017-01-28
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    2.Frequency of the third quartile class and – 2017-01-28
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    3. How many students are above the class? I could not get the answer for 2nd and 3rd question. – 2017-01-28
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    Well 2nd and 3rd follow from the first. 2 is just the y-value for whatever x-value you got for 1, and 3 is 25% by the definition of the 3rd Quartile class. – 2017-01-28
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    My best guess about why they have 1 as 30 is rounding up, it does look like it should be 25 or 26... – 2017-01-28
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    Let us [continue this discussion in chat](http://chat.stackexchange.com/rooms/52646/discussion-between-newton-and-vedvart1). – 2017-01-28
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    @Vedvart, for 1 it hasn't asked the exact value of third quartile but the class which is $20-30$ – 2017-01-28
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    The answer $30$ in the book is not for 1 BUT for the rest questions – 2017-01-28
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    We have different methods to find quartile. You can see https://en.m.wikipedia.org/wiki/Quartile – 2017-01-29
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    Have you given frequency table in question also. Or only diagram? – 2017-01-29
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    @@Kanwaljit Singh, Only the duagram. – 2017-01-29

1 Answers 1

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Upper quartile = $\frac{3N}{4}$

N = 120.

Upper quartile = 90th term

Through 90 on the y-axis draw a line parallel to y-axis meeting the curve at P.

From M draw a perpendicular on x-axis which meets it at Q.

You get value 25 on x-axis.

And class is 20-30.

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    @@Kanwaljit, and How many students are above the class? What is the answer for this – 2017-01-29
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    Draw graph and see its 25. – 2017-01-29
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    How's that? please explain--@@kanwaljit – 2017-01-29
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    Read my answer and draw diagram. – 2017-01-29