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?

8 round coins @ $15/11 = $120/11, so you are $1/11 short; if you change a round coin for a square one you'll get $121/11=$11.