Mathematicians calculate differently to Engineers

At a mundane ergonomics lecture, towards the back of the auditorium, the following discussion was recently overheard:

Engineer (E) to Mathematician (M):
(E) Let me ask you any question, and if you can't answer it you will give me $10. Ask me any question, and if I can't answer it I will give you $10.

The Mathematician tried to ignore the Engineer who persisted and bugged him with this question, that in the end he relented, especially as the offer from the Engineer had risen to $100 for an unanswered question by the Engineer.

(E): Ok, what is the maximum distance between the sun and the moon ?

The Mathematician withdrew, checked on his laptop various formulaes, cruised the internet and even searched through google, but had to relinquish $10 to the Engineer.
So it was now the Mathematician's go to ask a question.

(M) What goes up a hill with three legs and comes down with four?

The Engineer now was a bit baffled, but did not give up so easily and he too searched through the internet but could not come up with any solution. So he had to relinquish $100 to the mathematician. But of course he was not going to be left in the dark, and asked the mathematician:

(E) Well, what is the answer ?

The mathematician just shrugged his shoulders, gave the engineer $10 and left the lecture.