; This is not a trick question. 
; This is a real math problem, so don't say that a bus has no legs - and don't worry about  the bus driver!
; There are 7 girls in a bus.
; Each girl has 7 backpacks.
; In each backpack, there are 7 big cats.
; For every big cat there are 7 little cats.
; Question: How many legs are there in the bus?
(define (legs-in-bus girls backpacks big-cats little-cats)
  (define (girls-legs)
    (* 2 girls))
  (define (cats-legs cats)
    (* 4 cats))
  (define (backpack-legs)
   (+ (cats-legs big-cats) (cats-legs (* big-cats little-cats))))
  (+ (girls-legs) (* girls backpacks (backpack-legs))))