Prime numbers till 21 - 2, 3, 5, 7, 11, 13 17 19 so 8 of them..
Total ways to select 2 out of 21..=21*20

Probability=8*7/21*20=2/15

A
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1) Absolute modulus : http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
2)Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html
3) effects of arithmetic operations : https://gmatclub.com/forum/effects-of-arithmetic-operations-on-fractions-269413.html


GMAT online Tutor

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There are \(8\) prime numbers between \(1\) and \(21\), inclusive: \(2, 3, 5, 7, 11, 13, 17\) and \(19\).
So, there are 8C2 ways of selecting \(2\) numbers from these \(8\) prime numbers.
There are 21C2 ways of selecting \(2\) numbers from the \(21\) numbers from \(1\) to \(21\), inclusive.
Thus, the probability that the \(2\) selected numbers are prime numbers is 8C2 / 21C2 = ( 8*7 / 1*2) / ( 21*20 / 1*2 ) = 8*7 / 21*20 = 2/15.

Therefore, the answer is A.

Answer: A
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