Exercise 4.2.4:

```;; Fahrenheit->Celsius : number -> number
;; Example : (Fahrenheit->Celsius 32) -> 0
(define (Fahrenheit->Celsius t)
(* (- t 32) (/ 5 9)))

;; test-fahrenheit->celsius : number -> number
;; Example : (test-fahrenheit->celsius 32) -> true
;; Example : (test-fahrenheit->celsius 30) -> false
(define (test-fahrenheit->celsius t)
(= (Fahrenheit->Celsius t) 0))
```
```;; dollar->euro : number -> number
;; Example : (dollar->euro 1) -> 0.633
(define (dollar->euro x)
(* x 0.633))

;; test-dollar->euro : number -> number
;; Example : (test-dollar->euro 1) -> true
;; Example : (test-dollar->euro 0.633) -> false
(define (test-dollar->euro x)
(= (dollar->euro x) 0.633))
```
```;; triangle : number number -> number
;; Example : (triangle 2 3) -> 3
(define (triangle a h)
(* a (/ h 2)))

;; triangle : number number -> number
;; Example : (test-triangle 2 3) -> true
;; Example : (test-triangle 4 5) -> false
(define (test-triangle x y)
(= (triangle x y) 3))
```
```;; convert3 : number number number -> number
;; Example : (convert3 2 3 4) -> 432
(define (convert3 x y z)
(+ x (* 10 y) (* 100 z)))

;; convert3 : number number number -> number
;; Example : (test-convert3 2 3 4) -> true
;; Example : (test-convert3 3 4 5) -> false
(define (test-convert3 x y z)
(= (convert3 x y z) 432))
```