hmmm this reminds of the one where 3 guys go to an electrical store to but a tv costing £30, they each pay £10, manager makes a mistake tells the sales assitant its reduced to £25 , gives him the £5 to give back, he pockets £2, and gives the 3 guys £1 each.
10-1 3 times = £27
£2 in pocket! = £29 huh??
Just differnt wording same riddle!