Answer
Eight lots of 11p, 17p and 21p, and nine lots of 3p and 9p.
Reasoning
For three denominations of stamp I was asked to buy 8 of each, and for the other two denominations I had to buy 9 of each.
So I had to buy at least 8 of all five denominations.
This would be for a total of 8 x (3 + 9 + 11 + 17 + 21) = 488p = £4.88.
The remaining 12p has to buy the remaining 2 stamps, which must have been 3p and 9p.
Double-Checking
9 x 3 = 27
9 x 9 = 81
8 x 11 = 88
8 x 17 = 136
8 x 21 = 168
For a total of 27 + 81 + 88 + 136 + 168 = 500p = £5.
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Puzzle 198
Hidden in the grid below are three boys names. Starting at any point, trace out the path of a chess knight. The last letter of one name starts the next one. What are the names?