4. The maximum weight allowed in a service lift is satisfied exactly by 12 identical large packages or by 20 identical small packages. What is the greatest number of small packages that the lift can carry alongside nine large packages?
A 3
B 4
C 5
D 6
E 8
SOlUTION
C

Since the lift can take 12 large packages or 20 small packages, every three large packages can be replaced by five small packages. Therefore, the greatest number of small packages that can be carried with nine $(=12-3)$ large packages is 5 .