Source: Revised GRE PDF 2nd Ed. Section 5; #9 (p. 77)

11

One person is to be selected at random

One person is to be selected at random from a group of 25 people. The probability that the selected person will be a male is 0.44, and the probability that the selected person will be a male who was born before 1960 is 0.28. Quantity A: The number of males in the group who were born in 1960 or later Quantity B: 4 Quantity A is greater., Quantity B is greater., The two quantities are equal., The relationship cannot be determined from the information given.

5 Explanations

1

Robert Paul

I simply subtracted 0.44 (probability of all males) by 0.28 (probability of males born before 1960) to get 0.16 (probability of all males born AFTER 1960). Then simply multiplied 0.16 (All males born after 1960) and 25 (amount of people) to get 4.

0.44 - 0.28 = 0.16

0.16 * 25 = 4

Sep 18, 2014 • Comment

1

Imon Banerjee

Another way of thinking about prash's question might be, from the perspective of Probability. Probability = Success/Outcome.

P(Males born before 1960) = No. of males born before 1960/Total no. of people in the group

Here the probability of finding the Males before 1960 is given as 0.28. The number of people is 25, we have to find the No of males born before 1960.
Hence we multiply by 25 and not 11.

Aug 23, 2014 • Comment

1

sughan pandita

Thanks Chris:)

Aug 20, 2014 • Comment

4

prash k

Hi Chris,

for the 2nd step in this question -- why do you multiply the 0.28 by the total number of the group (which includes male + female) RATHER than 0.28 * the total number of males (which we calculated to be 11 - as per step 1).

I get lost there....

Jul 22, 2013 • Comment

Kevin Rocci, Magoosh Tutor

Hi Prash,

I think I can help with this.

We multiply 0.28 by the total number of people in the group and not the men in the group because 0.28 already represents the men in the group. That might be confusing. Let's say it another way.

Read the information in the prompt closely: 0.28 represents "a MALE who was born before 1960." That means that 28% of 25 people are male and were born before 196—two parameters.

If 0.28 only represented people born after 1960, and we were asked to find the males born before 1960, then we would follow the approach that you described (0.28 * the total number of males). But as it is, we only have to multiply 28% of the total group.

Does this make sense? I hope so!

Jul 23, 2013

2

Gravatar Chris Lele, Magoosh Tutor

Sep 27, 2012 • Comment

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