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3 guys have to pay 30 euros, they pay 10 euros each.
the other guy realises they should only pay 25 euros so he wants to give back 5 euros. then he wonders how he will split 5 euros in 3 even parts so he gives each person 1 euro back and keeps the other 2 euros.

so the 3 guys paid each 9 euros = 27 euros + the guy kept 2 euros = 29 euros... where's that other euro?