All you do is measure it with a ruler and for every 1 cm you get multiply it by 8.5ft. so for example if you got 6cm then you would multiply it by 8.5ft and your answer would be 51ft.
I mean it would be A and D because I don't know why they are both the same answer. If 10 books were sold yesterday, at LEAST 20 are needed to make the goal.