Enigma 998: Multiple purchases
9 August 2019
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From New Scientist #2153, 26th September 1998
The denominations of coins in circulation which are less than a pound are 50, 20, 10, 5, 2 and 1p.
Harry, Tom and I went into a shop recently and each made a purchase costing less than £1 (100p). The cost of each of these purchases was different. We each paid with a £1 coin and each received four coins in change — in each case the change due could not be given in fewer than four coins; but equally if we had paid the exact price for our purchases it would have been possible for each of us to have done so with four coins, but not with fewer.
The total cost of our three purchases was not only more than, but also an exact multiple of, the total amount of change than between us we received.
What did each of our purchases cost?