If x and y are positive integers and 7 + x + y + xy = 21, what is the value of x + y?  Pick:
A) 3
B) 5
C) 6
D) 8
E) 9
Try each answer choices.   For A: 7 + 3 + xy = 21; xy = 11 (impossible, 11 prime number. 1 + 11 doesn't equal 3)   For B: 7 + 5 + xy = 21; xy = 9 (no combination of xy = 9 and x + y = 5)   For C: 7 + 6 + xy = 21; xy = 8 (x + y = 6; x = 2, y = 4 or x = 4, y = 2)   For D: 7 + 8 + xy = 21; xy = 6 (no combination of xy = 6 and x + y = 8)   For E: 7 + 9 + xy = 21; xy = 5 (impossible, 5 prime number. 1 + 5 doesn't equal 9)   Therefore, answer 6.
<result>C</result>
If x and y are positive integers and 7 + x + y + xy = 21, what is the value of x + y?