# What is the greatest amount of postage you would not be able to pay…

What is the greatest amount of postage you would not be able to pay using only a combination of seven cent and seventeen cent stamps?

I have done a similar problem and got it correct but I am just wondering if there are other ways to do this. Please help me out thanks

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2
– Lopsy Dec 19 '12 at 23:47
How can we know whether there are "other ways" when we don't know what your way was? – Gerry Myerson Dec 20 '12 at 4:42

## 1 Answer

Suppose we can make $n$ cents. If we use two or more $17$s, we can replace two $17$s with five $7$s to make $n+1$. If we use twelve or more $7$s, we can replace twelve $7$s with five $17$s to make $n+1$. If we don't use two $17$s, and we don't use twelve $7$s, how large can $n$ be?

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