The overtime problem

According to one survey, teenage boys spend an average of 32% of their allowance on food, while teenage girls spend 26% on average. If a boy and a girl each receive n dollars, how much more does the boy spend on food? Write three functions: the first consumes an amount of allowance and returns how much a boy spends on food. The second consumes an amount of allowance and returns how much a girl spends on food. The third consumes an allowance amount and returns the difference between how much a boy and a girl spend on food.

; boy-food-budget : num (allowance) => num
; Ex: (boy-food-budget 10.00) => 3.20
; Ex: (boy-food-budget 0) => 0
(define (boy-food-budget allowance)
	( ... allowance ... ))

; girl-food-budget : num (allowance) => num
; Ex: (girl-food-budget 10.00) => 2.60
; Ex: (girl-food-budget 0) => 0
(define (girl-food-budget allowance)
	( ... allowance ... ))

; food-budget-difference : num (allowance) => num
; Ex: (food-budget-difference 10.00) => 0.60
; Ex: (food-budget-difference 0) => 0
(define (food-budget-difference allowance)
	( ... allowance ... ))

Now we have to come up with bodies for these functions. As suggested (and in keeping with the principle of "top-down design, bottom-up coding and testing"), we'll start with boy-food-budget and girl-food-budget, and only after these are tested and debugged will we go on to food-budget-difference.

(define (boy-food-budget allowance)
	(* allowance 0.32))

(define (girl-food-budget allowance)
	(* allowance 0.26))

Next we'll write the body for the food-budget-difference function. We already have functions to find how much each kid spends on food, so we just need to subtract them:

(define (food-budget-difference allowance)
	(- (boy-food-budget allowance)
	   (girl-food-budget allowance)))