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Brainteasers

Postage from 5 and 7 cent stamps

numbersfrobeniusdiophantinecoins

You have as many 5 cent and 7 cent stamps as you like.

  1. What is the largest amount of postage you cannot make exactly?
  2. How many amounts cannot be made at all?
  3. Give the general formula for two stamp values that share no common factor.
Show a hint

List the amounts from 1 upwards and see which can be made. Once you find five consecutive makeable amounts, everything above them can be made by adding 5s.

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