Yesterday my mother asked me to buy some stamps.
Stamps, in the land of BrainBashers, are available in 2p, 7p, 10p, 15p and 20p denominations.
For three types of stamp I was asked to buy five of each.
For the other two types of stamp, I was asked to buy six of each.
Unfortunately I forgot which I was supposed to buy five of and which to buy six of.
Luckily my mother had given me the exact money required to buy the stamps, £3.00 and the shopkeeper was able to give me the correct stamps.
Which stamps did I buy?
Five lots of 2p, 7p and 15p and six lots of 10p and 20p. The shopkeeper rightly figured that I required five lots of each of the stamps, which came to £2.70, he also knew I required two more stamps which added up to the difference. QED.