Show that all values of postage greater than 11 cents can be obtained using only 4 cent and 5 cent stamps.

Basis Step: A 12 cent postage can be formed with three 4 cent stamps.

Inductive Step: Assume that a postage of value n can be formed. Since n is greater than or equal to 12, there must be at least one 4 cent stamp present or at least three 5 cents stamp present.

If there are at least three 5 cent stamps we can remove them and replace them with four 4 cent stamps. Otherwise, remove one 4 cent stamp and replace it with one 5 cent stamp.

In this fashion we can always increase the postage by one cent.