Problem Solving

BTGmoderatorDC wrote:If is n is multiple of 5, and n=p^2*q where p and q are prime, which of the following must be a multiple of 25?

A. p^2
B. q^2
C. pq
D. p^2*q^2
E. p^3*q

OA D

Source: GMAT Prep

Let's test some values that satisfy the given conditions (n is multiple of 5, and n = p²q, where p and q are prime numbers)

How about: p = 2 and q = 5.
In this case, n = (2²)(5) = 20, and 20 is a multiple of 5, which satisfies the given condition.

Now plug p = 2 and q = 5 into the answer choices...
A) 2² = 4. This is NOT a multiple of 25. ELIMINATE
B) 5² = 25. This IS a multiple of 25. KEEP
C) (2)(5) = 10. This is NOT a multiple of 25. ELIMINATE
D) (2²)(5²) = 100. This IS a multiple of 25. KEEP
E) (2³)(5) = 40. This is NOT a multiple of 25. ELIMINATE

So, the correct answer is either B or D.

Let's try a new set of values.
How about: p = 5 and q = 2.
In this case, n = (5²)(2) = 20, and 20 is a multiple of 5, which satisfies the given condition.

Now plug p = 5 and q = 2 into the REMAINING answer choices...
B) 2² = 4. This is NOT a multiple of 25. ELIMINATE

NOTE: At this point, we can safely conclude that the correct answer is D.
But let's try D for "fun"...
D) (5²)(2²) = 100. This IS a multiple of 25.

Answer: D

Cheers,
Brent

BTGmoderatorDC wrote:If is n is multiple of 5, and n=p^2*q where p and q are prime, which of the following must be a multiple of 25?

A. p^2
B. q^2
C. pq
D. p^2*q^2
E. p^3*q

OA D

Source: GMAT Prep

A common phrase that is used on the GMAT is the word must. In this question, we are asked which of the following must be a multiple of 25. This means that one of our answer choices will always be a multiple of 25, no matter what. It is our job to determine which one, based on the given information.

We are given that n is a multiple of 5, n = (p^2)q, and that p and q are prime numbers.

Because n is a multiple of 5, a prime number, we know that either p or q is 5. Let's now analyze each answer choice to determine which one MUST (in all cases) be a multiple of 25.

A) p^2

If p = 3, then p^2 = 9 is not a multiple of 25. Answer choice A is not correct.

B) q^2

If q = 3, then q^2 = 9 is not a multiple of 25. Answer choice B is not correct.

C) pq

If p = 5 and q = 3 (or vice versa), pq = 15 is not a multiple of 25. Answer choice C is not correct.

D) (p^2)(q^2)

Regardless of which values we select for p and q, since we know that either p or q is 5, (p^2)(q^2) will always be a multiple of 25. If this is too difficult to see, let's use numbers.

If p = 5 and q = 3, (p^2)(q^2) = (25)(9) is a multiple of 25.

If p = 3 and q = 5, (p^2)(q^2) = (9)(25) is also a multiple of 25.

Answer choice D is correct.

For practice, let's analyze answer choice E.

E) (p^3)q

If p = 3 and q = 5, then (p^3)q = 135 is not a multiple of 25. Answer choice E is not correct.

Answer: D