### Brain Teasers

# Buying a Puppy

A man wishes to sell a puppy for $11. A customer wants to buy it but only has foreign currency. The exchange rate for the foreign currency is 11 round coins = $15, 11 square coins = $16, 11 triangular coins = $17. How many of each coinage should the customer pay?

### Answer

7 circular coins and 1 square coin.Let x be the number of circular coins, y be the number of square coins, and z be the number of triangular coins. Thus:

x*(15/11) + y*(16/11) + z*(17/11) = 11.

Multiplying by 11:

15x + 16y + 17z = 121.

From here we have to use trial and error or educated guessing. Personally I divided 121 by 15 to get 8 plus a remainder of 1. It was then obvious that x=7, y=1, and z=0 would solve the equation.

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