If x=y+4 and x=20-y, then x2-y2=
A
16
B
80
C
144
D
256
Correct Answer: 80
x + y = 20 x - y = 4 ( + ) 2x = 24 x = 12 Now, x - y = 4 y = x - 4 y = 12 - 4 = 8 x2 - y2 = 122 - 82 = 80