Question 983509
Let x = regular price of the cell phone. 

(a) Suppose only the 20% discount applies. 
.
f(x)=0.8x
.
(b) Suppose only the $50 coupon applies.  
.
g(x)=x-$50
.
(c) If you can use the coupon and the discount, then the purchase 
price is either (f o g)x or (g o f)x.  Which composition gives the lower price? 
.
f(g(x))=0.8(x-$50)
g(f(x))=0.8x-$50 This is the lower price.