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?

Question

Mathematics

Dante Whitney

22 February, 04:15

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?

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Answers (1)

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Ricardo Robles · Answer 1

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

Step-by-step 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.