How smart are you

[Tread]

Another one that I like:

Three men bought an oven for their flat share for 300?. But the salesman realized the oven should have been only 250?. So he sent the trainee and told him to give 50? back to the men. The Trainee cheated and gave back 10? to each, 30? in sum, and hold 20? for himself.
So every man paid actually 90?, together they paid 270? (90? * 3 = 270?). The trainee has 20?.
270? + 20? = 290?
It was 300?, where are the remaining 10??



@ shadows and Kakariko for Question #2:
Change the word doctor to nurse, read again, and change nurse to doctor. The magic of clich? Occupations.