1) At the flea market, a seller sells a shelf for 80 Euro that he had bought for 70 Euro. Then he changes his mind, buys back the shelf for 90 Euro and resells it for 100 Euro. How much did he earn in the end?
(A) Nothing
(B) 10 Euro
(C) 20 Euro
(D) He lost 10 Euro

(E) None of the above.
1) The answer is (C)

The seller earns 80 - 70 Euros from the first sale and 100 - 90 Euros from the second sale. His total profit will therefore be $10+10$ Euros.