In the early days of Canada Post the only stamps available were 5-cent
and 11-cent stamps.  In fact, when people were out of stamps they
would glue nickels and 11-cent pieces (cobalts) to the outside of
envelopes, using a mixture of wolverine tallow and balsam gum.

Can you find a minimal possible number N such that for every natural 
number n>N, postage of n-cents can be made with 5-cent and 11-cent stamps?  
If so, for whatever value of N you come up with: 
(a) Prove, using complete induction, that for any n>N, postage of n
cents can be made with 5-cent and 11-cent stamps.

(b) Prove that it is impossible to make postage of N cents using
5-cent and 11-cent stamps.  Don't assume that induction is the right
tool to use for this part.