Yesterday I was asked to buy some stamps.
In the land of BrainBashers, stamps are available in denominations of 3p, 9p, 11p, 17p, and 21p (just these five different values).
For three denominations of stamp I was asked to buy eight of each, and for the other two denominations 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 I had been given the exact money required to buy the stamps, £5.00, so the shopkeeper was able to work out the stamps I needed.
Which stamps did I have to buy?