D...

I plugged in starting w/ the middle answer...product must total 6 (factors of 1,6 and 3,2)

1 + 3 + 2 +(3*2)=12
1 + 1 +6 + (6*1)=14...NO

Go to the next biggest since the answer was not quite large enough...

8 (factors of 1,8 and 2,4)

1 + 4 + 2 + (4*2)=15 DING!
to follow through 1 + 1 + 8 + (8*1)=18 No.

Maybe not the best way, sorry if it isn't descriptive enough, that was the easiest way I saw.

wouldnt it be C then...2+4=6

I think the right way is as Hovey25 describe it, but we are not looking for the prOduct, we are looking for the sum so to get a 6 you can use 4 and 2, the right answer is C.

You are right, careless error...

Sorry

1+x+y+xy = 15
(1+x) + y(1+x) = 15
(1+x)(1+y)=15

we have 2 options
5*3 or 15*1
15*1 wont work, since y will be zero (but x & y are +ve integers)

1+x = 5
1+y = 3

so X+Y =6

HTH