Hi fattty,
This question can be solved by TESTing VALUES and Number Properties.
We're told that R and S are POSITIVE INTEGERS. We're asked if (R)(S) is a multiple of 25? This is a YES/NO question.
1) R - S is a multiple of 5
This Fact tells us that EITHER both variables are multiples of 5 OR both variables are NOT multiples of 5.
IF....
R = 6
S = 1
(6)(1) = 6 and the answer to the question is NO.
IF....
R = 10
S = 5
(10)(5) = 50 and the answer to the question is YES.
Fact 1 is INSUFFICIENT
(2) (R)(S) is a multiple of 5
This Fact tells us that AT LEAST ONE of the two variables is a multiple of 5.
IF....
R = 5
S = 4
(5)(4) = 20 and the answer to the question is NO.
IF....
R = 10
S = 5
(10)(5) = 50 and the answer to the question is YES.
Fact 2 is INSUFFICIENT
Combined, we know that...
R - S is a multiple of 5
(R)(S) is a multiple of 5
From Fact 2, we know that AT LEAST one of the two variables is a multiple of 5. From the Fact 1, we know that IF one of the variables is a multiple of 5, then the other one MUST be also. Combining these two Facts proves that BOTH variables are multiples of 5; when you multiply two multiples of 5 together, you end up with a larger number that is a multiple of 25. Thus, the answer to the question is ALWAYS YES.
Combined, SUFFICIENT
Final Answer:
GMAT assassins aren't born, they're made,
Rich
_________________