Data Sufficiency

How to solve this question?

Hi codesnooker,

The answer is D.

(1) creates 2 equations with 2 variables, so you can resolve the problem
x + y =77
x = y + 1

(2) states that both x & y share the same tens digit.
The only tens digit the can share is 3 (2 would sum less than 40 and 4 would add up more than 80).
Then you have to separate the number 17 in two one digit numbers (to add them to x and y) and this number can only be separated as 8 + 9.
Thus you have two solutions x=38 & y = 39 or x=39 & y = 38.
But since the question asks the value of x*y you don't really care which one is 39 and which is 38.


I hope to have been clear (I'm still improving my English)

It's a height of my carelessness. I had done wrong division while dividing 77 with 2. That's weird 100% weird.

Thanks Augusto for opening my eyes.