Question

Imagine the United States only had two postage stamps, a 3 cent stamp and a 7 cent stamp. If you can put any number of stamps on an envelope that you want, what amounts of postage can and cannot be paid for?

Answer 1

• not possible: 1, 2, 4, 5, 8, 11
• possible: all others

Explanation:

Obviously, all multiples of 3 and 7 can be paid.

1 cent can be added by adding a 7-cent stamp and taking away two 3-cent stamps. 2 cents can be added by adding three 3-cent stamps and taking away one 7-cent stamp. Hence any value more than 7 +2(3) = 13 cents can always be accommodated.

Amounts that are impossible:

1, 2, 4, 5, 8, 11 cents

All other positive integer amounts are possible.