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Schools:Indian School of Business (ISB) - Class of 2013

GMAT 1: 730 Q49 V41

WE: Other (Pharmaceuticals and Biotech)

Re: How many times must a fair six sided die be rolled so that [#permalink]

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01 Sep 2010, 11:01

Let n be the number of times the dice is rolled to ensure that the probability of a 4 is at least 1/4.

Thus, n*(probability of rolling a 4 in one turn)>= 1/4 n*(1/6)>= 1/4 n>6/4---->n>= 1.5 Hence "minimum number of times to ensure that the probability that a 4 is rolled is at least 1/4" >= 1.5 = 2 (since we cannot have 1.5 rolls)

Re: How many times must a fair six sided die be rolled so that [#permalink]

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04 Feb 2013, 03:34

Cornelius wrote:

Let n be the number of times the dice is rolled to ensure that the probability of a 4 is at least 1/4.

Thus, n*(probability of rolling a 4 in one turn)>= 1/4 n*(1/6)>= 1/4 n>6/4---->n>= 1.5 Hence "minimum number of times to ensure that the probability that a 4 is rolled is at least 1/4" >= 1.5 = 2 (since we cannot have 1.5 rolls)

Ans B

By this logic, in 6 throws, you have a probability of getting 4 as 1. I doubt if your approach is correct. And in 7 and above throws the probability to get 4 will be more than 1. I dont know if I am missing something
_________________

Concentration: Entrepreneurship, International Business

GMAT 1: 730 Q50 V39

GPA: 3.2

WE: Education (Education)

Re: How many times must a fair six sided die be rolled so that [#permalink]

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16 Feb 2013, 16:27

How I look at it is that since the number of throws is lesser, there is lesser chance of all numbers coming up. Hence, 4 has a chance of coming up more than 1/4 times in two throws.

Another way to look at it is that in two throws only two numbers can come up and the chance of 4 coming up once in two trials will be higher than 1/6

4 not 4 not 4 4

1/6*5/6+5/6*1/6 = 25/36>1/4

sagarsingh wrote:

Cornelius wrote:

Let n be the number of times the dice is rolled to ensure that the probability of a 4 is at least 1/4.

Thus, n*(probability of rolling a 4 in one turn)>= 1/4 n*(1/6)>= 1/4 n>6/4---->n>= 1.5 Hence "minimum number of times to ensure that the probability that a 4 is rolled is at least 1/4" >= 1.5 = 2 (since we cannot have 1.5 rolls)

Ans B

By this logic, in 6 throws, you have a probability of getting 4 as 1. I doubt if your approach is correct. And in 7 and above throws the probability to get 4 will be more than 1. I dont know if I am missing something

Re: How many times must a fair six sided die be rolled so that [#permalink]

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14 Oct 2017, 20:32

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