As you go around the supermarket, try to keep a tally of how much the bill will come to. Not exactly, just roughly. E.g. "Two item, about 70p each, one item, about £1.50, so that's a little under £3. Next item is a little over £1, so the total's about £4...." you get the idea. See if you can get the total right to within, say 5%.
FFF
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[Edited because my mental maths was wrong!
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