Exercise 3.1.3: (back)

;; How to design a program
> (profit 5.00)
415.2

> (profit 4.00)
889.2

> (profit 3.00)
1063.2
;; How not to design a program
> (profit 5.00)
415.2

> (profit 4.00)
889.2

> (profit 3.00)
1063.2