If 3 < x < 100, for how many values of x

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If 3 < x < 100, for how many values of x

by kamalj » Mon Dec 18, 2017 9:22 am
If 3 < x < 100, for how many values of x is x/3 the square of a prime number?

(A) Two
(B) Three
(C) Four
(D) Five
(E) Nine

OAB

Please explain.

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by Brent@GMATPrepNow » Mon Dec 18, 2017 9:45 am
kamalj wrote:If 3 < x < 100, for how many values of x is x/3 the square of a prime number?

(A) Two
(B) Three
(C) Four
(D) Five
(E) Nine

OAB

Please explain.
We want values of x (where 3 < x < 100) such that x/3 is the square of a prime number.
So, let's start checking squares of prime numbers.
Some prime numbers are 2, 3, 5, 7, 11, etc

2² = 4 and (3)(4) = 12. So, x = 12 meets the given conditions.
3² = 9 and (3)(9) = 27. So, x = 27 meets the given condition
5² = 25 and (3)(25) = 75. So, x = 75 meets the given conditions.
7² = 49 and (3)(49) = 147. No good. We need values of x such that 3 < x < 100

So, there are exactly 3 values of x that meet the given conditions.
Answer: B

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
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