The largest unmakeable postage

You have unlimited stamps worth 55 and 2121 (cents). What is the largest value that cannot be formed as a non-negative combination 5a+21b5a + 21b?

Show hints (2)+
  1. For coprime m,nm,n, the Frobenius (largest non-representable) number is mnmnmn-m-n.
  2. 521521=795\cdot21-5-21=79.

Answer

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79

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Asked at: Jane Street, SIG

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