i have £10 my friend has £10 and my other friend has £10 so in total we have £30
we go into a shop and want to buy a game i ask the shop keeper how much it is he says its £30 so we each give him £10 and walk off
the shop keeper then realized it was £25 not £30 so he gets 5 £1 coins and runs after us but he then realized he cant split £5  between 3 people so he keeps 2 for him self and gives us £1 each so in the end we spent £9 each and there were 3 of us so thats £27 in total

but wait a second if the shop keeper kept £2 and we spent £27 that only adds up to £29 what happened to the last pound?