Question 458879
all you have to do is solve for y:
x - x(y) = z
x(1-y) = z
(1-y) = z/x
-y = z/x - 1
y = 1 - z/x
.
that is, percent reduction 
1 - "net pay"/"gross pay"
.
1 - 531/600
1 - .885
.115
.
Multiply the above to give the "percentage":
.115 * 100 = 11.5%