(Warning, this solution requires a lot of assumptions)
Well, assuming A and B are two points on a wire, running left to right in front of you, you could have two solutions;
1. The fault could be one third of the way between points A and B. Now, from which point you would measure this would be arbitrary...it would either be 25 feet from A, or 50 feet from B.
2. The fault could also be 75 feet "left" of A. The problem didn't specify, among other things, the resistivity of the wire. It also did not specify the wire was only between A and B.
My opinion is that even if you get this problem right, you come away feeling a little stupider for even having answered it...but if I was taking a test and it counted, I would give the "50" answer and pray.
This whole things smacks of some teachers giving multiple choice exams full of "best" answers. Hated those guys in school.