Have you heard it before or did you work it out?

. All credit if you worked it out lol.
There's £30 originally, £25 is in the till, divide the £25 by 3 and they've paid £8.33 each then give them a £1 pack each and they've paid £9.33 as D-A-L said. The other £2 is then with the thieving waiter.
Its clever how the way that puzzle is phrased can catch you out