I don't know how to do or start this proof type of question. There are 2p and 5p stamps in a box. By considering even and odd amounts, prove that any postage value can be made of 4p or greater.
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Clearly we can make 4p with 2 2p stamps.
Now, for n = 4 + 2k, we just add k additional 2p stamps, and there we go.
On the other hand, for n = 4 + (2k + 1) = 5 + 2k, we add k 2p stamps to a 5p stamp, and boom!
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