Hi Laura,
Cluin is too clever for me. I think that was an answer for left-brained people but I'm right-brained. I suspect you are too!
The way I see it, they each paid 10 pound (I only have a $ sign on my keyboard), they were each given 1 pound back, that makes 27 pound, but the shopkeeper only got 25 pound.
Where did the other 2 pound go? That was what the assistant pocketed! Does that make sense now?
Robyn