Okay so listen closely
3 friend went to a hotel, and receptionist tells them that 1 room for all 3 of them costs 30€, so, normally they split the price and each of them gives 10€ for a room. Latter during the day receptionist recalls thar room price is actually 25€. So he hands 5€ to the bellboy and told him to return the money. While he was going to the room, he didnt know how to split 5€ to each of them, so he took 2€ for him and gave to each of friends 1€.
So aftermath is that each of friends paid 9€, and 2€ are kept by bellboy.
So each of friends pid 10-1=9€
9€+9€+9€+2€=29€
Where is 1€?