The product of the squares of two positive integers is 400. How many pairs of positive integers satisfy this condition?
Answers.
A) 0  B) 1  C) 2  D) 3  E) 4

Rationale: first break down 200 into 20 * 20 and further into the prime factors 2 * 2 * 5 * 2 * 2 * 5 . now we are looking for all the possible pairs ( 2 numbers ) of squares whose product results in 400 . 1 st : 2 ^ 2 * 10 ^ 2 ( i . e . the first two 2 ' s and two times 2 * 5 = 10 ) 2 nd : 4 ^ 2 * 5 ^ 2 ( i . e . two times 2 * 2 = 4 = 4 ^ 2 and 5 ^ 2 ) . 3 rd : 1 ^ 2 * 20 ^ 2 ( i . e . two times 2 * 2 * 5 and 1 ^ 2 = 1 ) answer d .
Answer: D