Suppose half the people on a bus exit at each stop and no additional passengers ... ...

Suppose half the people on a bus exit at each stop and no additional passengers board the bus. If on the third stop the next to last person exits the bus, then how many people were on the bus?
(A)    20
(B) ...
(C) ...
(D) ...
(E) ...

*This question is included in Nova Math - Problem Set C: Substitution (Plugging In)