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Susan flipped a fair coin N times. What fraction of the flips came up

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synecdoche
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Susan flipped a fair coin N times. What fraction of the flips came up [#permalink] New post   Updated on: 12 Dec 2018, 04:44
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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

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Hi,

When you get a question like-

"X flipped a coin N times. What fraction of the flips came up heads?
(1) N=10
(2) 3/8N came up tails. "

Do you just answer B or do you try to be extra smart and ask whether it's a fair coin? (And hence, answer E)
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Official Answer
B

Originally posted by synecdoche on 11 Jun 2012, 19:29.
Last edited by Bunuel on 12 Dec 2018, 04:44, edited 2 times in total.
Edited the question.
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Bunuel
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Re: Susan flipped a fair coin N times. What fraction of the flips came up [#permalink] New post   12 Jun 2012, 05:10
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synecdoche wrote:
Hi,

When you get a question like-

"X flipped a coin N times. What fraction of the flips came up heads?
(1) N=10
(2) 3/8N came up tails. "

Do you just answer B or do you try to be extra smart and ask whether it's a fair coin? (And hence, answer E)


The GMAT will always make clear whether the coin is fair or not (for example it can be given that the probability of heads is 0.4 and the probability of tails is 0.6). So, you shouldn't worry about that issue.

Having said that, Kaplan also makes it clear in the original question, which is:
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

Clearly, the first statement is not sufficient. Even though expected number of heads is 12 out of 24 but we don't know how many heads there were actually.

As for the second statement: since 3/8 of the flips came up tails then the remaining 5/8 of the flips must have been heads (provided N>0).

Hope it's clear.
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LogicGuru1
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Susan flipped a fair coin N times. What fraction of the flips came up [#permalink] New post   Updated on: 30 Aug 2020, 06:57
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Suson flipped a fair coin N times. What fraction of the flips came up heads?

(1) N = 24
SO close yet so far. Because we know that the coin is a fair therefore the probability of getting heads is 50 % but probability does not mean "Definitely".

Since the question is asking "FRACTION" of HEAD which is \(\frac{# of HEAD}{# of TOTAL-FLIPS}\).

we cannot get this value because we dont know how many times the coin actually came as HEAD. All we know that its probability of gettting a head was 50% in each of the 24 tosses but how many times it was actually head is not known

INSUFFICIENT

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

Number of times that tail came up = \(\frac{3N}{8}\)

Therefore total number of times head came up = \(\frac{5N}{8}\)

Fraction of head = \(\frac{5N}{8}*\frac{1}{N} =\frac{5}{8}\)of total

SUFFICIENT

ANSWER IS B

Originally posted by LogicGuru1 on 05 Aug 2016, 06:03.
Last edited by LogicGuru1 on 30 Aug 2020, 06:57, edited 1 time in total.
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CEdward
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Re: Susan flipped a fair coin N times. What fraction of the flips came up [#permalink] New post   11 Feb 2021, 19:04
C-trap problem

St1 - N = 24
Clearly insufficient.

St2 - t = 3/8N
It follows that heads = 5/8N
1 - 3/8 = 5/8
Sufficient.

B
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siddharth30
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Re: Susan flipped a fair coin N times. What fraction of the flips came up [#permalink] New post   06 Dec 2022, 09:01
CEdward wrote:
C-trap problem

St1 - N = 24
Clearly insufficient.

St2 - t = 3/8N
It follows that heads = 5/8N
1 - 3/8 = 5/8
Sufficient.

B


can you pls explain how N was cancelled??
Thank you
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Re: Susan flipped a fair coin N times. What fraction of the flips came up [#permalink]
06 Dec 2022, 09:01
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Bunuel
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