Three colleagues have a nice lunch together, and the bill is €25.
The three colleagues pay €10 each, and the waiter gives the total amount to the cashier.

The cashier hands back €5 to the waiter, but the waiter can’t split €5 three ways, so he gives the colleagues one euro each and keeps 2 euro as a tip.

They all paid €10 and got €1 back: €10 – €1 = €9

There were three of them 3 x €9 = €27

If they paid €27 and the waiter kept €2 (€27 + €2 = €29)
…where did the other euro go? (€30 – €1 = €29)