;; Updated Monday Sept 13th. 

;; Contract: number -> number

;; Purpose: determine a yearly interest rate based on amount
;; The interest rate is 0.04 for amounts between 0 and
;; 1000 (incl), 0.045 for amounts between 1000 and 5000 (incl),
;; and 0.05 for larger amounts.
;; Examples: (interest-rate 500) -> 0.04
;; (interest-rate 1000) -> 0.04
;; (interest-rate 10000) -> 0.05
(define (interest-rate m)
 (cond
   [(< m 0) 'NegativeAmount]
   [(>= 1000 m) .04]
   [(>= 5000 m) .045]
   [else .05]
   ))


;; Tests
(check-expect (interest-rate 500) 0.04)
(check-expect (interest-rate 1000) 0.04)
(check-expect (interest-rate 3000) 0.045)
(check-expect (interest-rate 10000) 0.05)
(check-expect (interest-rate 0) 0.04)
(check-expect (interest-rate -500) 'NegativeAmount)



;; Write a function that, given an amount of money,
;; computes the interest on that amount that accumulates in a year