1. Direct proof

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.

2. Re: Direct proof

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!