Any amount of postage greater than or equal to 12 cents can be built
using only 4-cent and 5-cent stamps.
__Base cases:__
n=12: 12=4+4+4
n=13: 13=4+4+5
n=14: 14=4+5+5
n=15: 15=5+5+5
__Inductive step:__ If any amount of postage r, 12<=r<=k, can be built
using only 4-cent and 5-cent stamps, then postage k+1 can be built using only
4-cent and 5-cent stamps.
__Proof__: Cases k=12, k=13, k=14, and k=15 were proved in the base case,
so here we can assume that k+1 >= 16. By the inductive hypothesis, we know
that the postage for k-3 can be formed using 4-cent and 5-cent stamps since
(k+1)>=16 => (k-3)>=12. Now take the postage for k-3 and add a 4-cent stamp;
this gives the postage for k+1.