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by GmatKiss » Sun Oct 16, 2011 6:33 am
Susan flipped a fair coin N times. What fraction of the flips came up heads?
(1) N = 24
(2) The number of flips that came up tails was N .

IMO: B
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by Brent@GMATPrepNow » Sun Oct 16, 2011 7:00 am
GmatKiss wrote:Susan flipped a fair coin N times. What fraction of the flips came up heads?
(1) N = 24
(2) The number of flips that came up tails was N .

IMO: B
Statement 1:
Although we might expect that 1/2 of the flips turned up heads, we don't know this.
INSUFFICIENT

Statement 2:
If N of the N flips came up tails, then 0 of the N flips came up heads.
So, the fraction of the flips that came up heads = 0/N = 0
SUFFICIENT

Answer: B

Aside: this answer assumes that N>0. If we allow for N to equal 0, then I suppose the answer would be C.

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by leonswati » Sun Oct 16, 2011 7:20 am
Even I chose the answer to be C... How do we know if N> 0 or not....?
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by leonswati » Sun Oct 16, 2011 7:24 am
This is from Kaplan test ... I checked the source and found this as the right question:

Susan flipped a fair coin N times. what fraction of the flips came up heads?

1) N = 24
2) The number of flips that came up tails was 3/8(N)


According to this question the answer has to be B.
Here N has to be a multiple of 8.
So let N be 8,
then the number of flips that came up tails is 3
therefore no of flips that has to be head is 8-3 = 5
fraction = 5/3

So let N be 16,
then the number of flips that came up tails is 6
therefore no of flips that has to be head is 16-6 = 10
fraction = 10/6 = 5/3

We will get the same fraction.. Therefore answer is B
Last edited by leonswati on Sun Oct 16, 2011 7:29 am, edited 1 time in total.
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by Brent@GMATPrepNow » Sun Oct 16, 2011 7:24 am
leonswati wrote:Even I chose the answer to be C... How do we know if N> 0 or not....?
Good question - there seems to be some ambiguity here.
Presumably this is not an official question, otherwise it would remove any ambiguity.
For example, we might add some proviso like "(where N>0).
I don't think it's necessary to try guess what the answer would be if this question were to appear on the GMAT, since it wouldn't appear (in its current form) on the GMAT.

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by leonswati » Sun Oct 16, 2011 7:42 am
This is from Kaplan test ... I checked the source and found this as the right question:

Susan flipped a fair coin N times. what fraction of the flips came up heads?

1) N = 24
2) The number of flips that came up tails was 3/8(N)


According to this question the answer has to be B.
Here N has to be a multiple of 8.
So let N be 8,
then the number of flips that came up tails is 3
therefore no of flips that has to be head is 8-3 = 5
fraction = 5/3

So let N be 16,
then the number of flips that came up tails is 6
therefore no of flips that has to be head is 16-6 = 10
fraction = 10/6 = 5/3

We will get the same fraction.. Therefore answer is B
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