(define factorial
  (lambda (n)
    (if (zero? n) 1
        (* n (factorial (- n 1))))))

; Number of ways to look at paths between 100 cities
(define paths (factorial 25))
; If we can do a trillion paths a second
(define seconds (/ paths 1000000000000))
; How many minutes?
(define minutes (/ seconds 60))
; How many hours?
(define hours (/ minutes 60))
; How many days?
(define days (/ hours 24))
; How many years?
(define years (/ days 365.24))