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## Homework Statement

If you have only 5 and 7 cent stamps you can make perfect change of a 5,7,10,15,17, etc. cent letter. You cannot mail a 1,2,3,4,6,8,9,11,etc. cent letter. What is the greatest amount that you cannot make perfect change for?

**2.Attempt at a Solution**

I'm pretty confident that the answer is 23 cents, however I want to be able to prove that that is in fact the absolute greatest amount for all possible combinations. Thanks for the help