I am little skepticle about the question. The question is silent about the order of outcome i.e. an outcome of (21 and 23) and (23 and 21). IMO, an outcome of (21 and 23) is the same as (23 and 21) because the order here does not matter.
Total numbers = 20, 21, 22, 23, 24,25, 26, 27, 28, & 29. 10 numbers
2 out of 10 = 10c2 +10 = 55
primes = 23 and 29
multiple of 3 = 21, 24, and 27
Getting a prime and a multiple of 3 = 2x3 = 6
The prob = 6/55 = 11%
bipolarbear wrote:
What is the probability that one of the two integers randomly selected from range 20-29 is prime and the other is a multiple of 3?
(The numbers are selected independently of each other, i.e. they can be equal)
(C) 2008 GMAT Club -
Probability and Combinations#10 * 0.06
* 0.12
* 0.15
* 0.18
* 0.20
Prime integers: 23 and 29. Multiples of 3: 21, 24, and 27.
The probability that the first number is prime while the second is a multiple of 3 = \frac{2}{10} \frac{3}{10} = 0.06.
The probability that the first number is a multiple of 3 while the second is prime = \frac{3}{10} \frac{2}{10} = 0.06.
The probability that one of the two integers is prime and the other is a multiple of 3 = 0.06 + 0.06 = 0.12.
The correct answer is B.
Why are they added together and not multiplied together?
_________________
Verbal: new-to-the-verbal-forum-please-read-this-first-77546.html
Math: new-to-the-math-forum-please-read-this-first-77764.html
Gmat: everything-you-need-to-prepare-for-the-gmat-revised-77983.html
GT
close
