A rock band's tour bus, mass M, is accelerating away from a stop sign at rate a

Question:

A rock band's tour bus, mass M, is accelerating away from a stop sign at rate a when a piece of heavy metal, mass M/5, falls onto the top of the bus and remains there.
a. Show that the bus's acceleration is now 5/6a.
b. If the initial acceleration of the bus is 1.2 m/s2, show that when the bus carries the heavy metal with it, the acceleration will be 1.0 m/s2.