sol-2.5.tex

We can indeed represent a pair of nonnegative integers~$a, b$ as the product $2^a 3^{\smash{b}}$, because the function $(a, b)\mapsto 2^a 3^b$ is injective, by the {\em fundamental theorem of arithmetic}. 
Here are possible definitions for `cons`, `car`, and `cdr`:
\begtt\scm
(define (cons a b)
  (* (expt 2 a) (expt 3 b)))

(define (car p)
  (define (iter p result)
    (if (= 0 (remainder p 2))
        (iter (/ p 2) (inc result))
        result))
  (iter p 0))

(define (cdr p)
  (define (iter p result)
    (if (= 0 (remainder p 3))
        (iter (/ p 3) (inc result))
        result))
  (iter p 0))
\endtt