I had lunch with a couple of colleagues.
The bill came to £25, so we each gave the waitress £10, a total of £30.
As the £5 change couldn't be tidily divided between us, we asked for £1 each in change and told the waitress to keep the odd £2 as a tip.
We ended up paying £27 each for meal ((3 x £10) - (3 x £1 in change)) + £2 tip = £29 in total.
Never did figure out where the missing £1 went from the original £30.
(It's an old riddle, but still good for the odd argument.)