The £27 the friends paid is £25 to the hotel plus the porter’s £2, so adding his £2 on again counts it twice. The sum that makes £30 is £27 paid plus £3 handed back.
Three friends pay £30 for a hotel room, £10 each. The manager realises the room is only £25 and sends the porter up with £5. The porter can’t split £5 three ways, so he pockets £2 and gives each friend £1 back. So each friend has paid £9, which makes £27, and the porter has £2, which makes £29. Where is the missing pound?
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Should the porter’s £2 be added to the £27, or taken away from it?
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There isn’t one
Source: Traditional; an early form in Evelyn August, The Black-Out Book, 1939