#math
I recently watch a similar puzzle
where a man was selling something for 9$
received his change from a 20$ note
11$ then decided to buy another item for 2$ and gave back only one dollar, the cashier was surprised, and the customer said, yes 9+2 = 11 look at what you have in your hand, and sure enough he had 11$ (the 10$ note and the 1$)
Maybe you saw that one