3 friends went thirds in a new toaster which retails for $30 ($10 each).
The salesman later found the price should have been $25 so gave them each a $1 refund so it cost $9 each and kept the other $2.
$9 x 3 = $27 (what was paid by the 3 people)
$27 + $2 (what the salesman kept) = $29
But the toaster originally cost $30 so where did the extra $1 go?