Distance between point A and point B is D. You jump into your Audi R8 and travel the distance D with a constant speed S.
Considering that you want to return from B to A and you want your average speed (roundtrip) to be 2*S, what is speed you should have on the return trip?
You waste no time in B and return right away.
Source and solution will be posted after you take a stab at this problem :)
Later edit:
I eventually got the solution both offline and on Facebook.
For reference, here is the source (problem number 8):
http://rinkworks.com/brainfood/p/math1.shtml