DS Archive | Re: If x and y are positive integers such that the product of x

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

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I get that just from the question stem itself, you know either x or y must be equal to 1, thus B cannot be the answer. I can't figure out how you can tell the units digit of 9 to the Y just from the statement one.

Help?