Stamp buying riddle
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Yesterday John’s mother asked him to buy some stamps. Stamps, in the land of Puzzlefry, are available in 3p, 9p, 11p, 17p and 21p denominations.
For three types of stamp John was asked to buy eight of each. For the other two types of stamp, he was asked to buy nine of each.
Unfortunately he forgot which he was supposed to buy eight of and which to buy nine of.
Luckily his mother had given him the exact money required to buy the stamps, £5.00 and the shopkeeper was able to give him the correct stamps.
Which stamps did he buy? 
Answer: He bought 9 of 3p and 9p and 8 of 11p, 17p and 21p stamp dominations.
Solution: Sum of 8 of all stamp dominations is 8*(3+9+11+17+21) = 488, which means that there are 12p left and only way is to use one 3p and one 9p. So he bought 9 of 3p and 9p and 8 of 11p, 17p and 21p stamp dominations ( 9*(3+9)+8*(11+17+21) = 108 + 392 = 500 ).
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