Stamps

Yesterday my mother asked me to buy some stamps. Stamps, are available in 3p, 9p, 11p, 17p and 21p denominations. For three types of stamp I was asked to buy eight of each. For the other two types of stamp, I was asked to buy nine of each. Unfortunately I forgot which I was supposed to buy eight of and which to buy nine of. Luckily my mother had given me the exact money required to buy the stamps, £5.00 and the shopkeeper was able to give me the correct stamps. Which stamps did I buy? Eight lots of 11p, 17p and 21p and nine lots of 3p and 9p. The shopkeeper rightly figured that I required eight lots of each of the stamps, which came to £4.88, he also knew I required two more stamps which added up to the difference. QED.