The bill of a light dinner for three friends is 25 €. Each one gave 10€. So they gave 3*10=30€ in total.
The waiter gave them back 5€ and they gave him a tip of 2€. So each one took back 1€. They paid
3(10-1)=3*9=27€. If we add the given tip we have 27+2=29€ in total. Where is the missing euro?