If x and y are positive integers such that the product of x and y is prime, what is the units’ digit of 7^x + 9^y?
(1) 24 < y < 32
(2) x = 1
(1) 24 < y < 32
(2) x = 1
if the product of xy=prime then either x or y has to be 1 and the other one must be prime. Otherwise the product will not be prime.
1) 24<y<32: y = 29 or 31, x = 1
7^1 + 9^29 : Unit digit = 7 +9=16, Unit digit = 6
7^1 + 9^31 : Unit digit = 7 +9=16, Unit digit = 6
Unit digit of 9 to the power of any positive odd integer is 9.
Sufficient
2) x=1, we have no information on the value of y.
Not Sufficient
IMO: answer = A[/spoiler]
