Hermione wrote:

If x and y are positive integers such that the product of x and y is prime, what is the unitsâ€™ digit of 7xx + 9y?

(1) 24 < y < 32

(2) x = 1

--> Here, i'm not sure if 7xx means (7)(x)(x) or xx are the last two digits of a 3 digit number 7_ _

In either case, I get 2 sets of diff units digits.

St 2 is redundant. If product of 2 nbrs is prime, then one of them has to be 1 and the other prime. S1 resolves that by saying y is between 24 and 32 which gives us 2 primes, 29 and 31 for the value of y.

For 7XX +9Y, I get 972 and 990

For 7X^2+9Y, I get 268 and 286

Hence E.

(Hope I havent made any calc mistakes)