The answers are:
ƒ(x) = x - .15x
Sale = Original - .15(Original)
y = .85x
Let sale price be f(x) and x be the original price. Discount was 15% = 0.15
f(x) = x - 0.15x
If f(x) is sale and x is the original, then:
Sale = Original - .15(Original)
Let sale price be y and original price x:
y = x - 0.15x
y = 1 * x - 0.15 * x
y = (1 - 0.15) * x
y = 0.85x