this problem can provide some valuable practice -- especially in using the number-picking method described by the poster above -- but it's not well written (and definitely not written as conscientiously as the official problems are).

given the wording of this problem, the man could just as well reach his destination an hour early by, say, traveling at exactly the same speed but leaving four hours earlier, or by starting from a point that's closer to the destination. (as transcribed, the problem says nothing about the time or point of departure, so there's no good reason to assume that either is the same as in the original situation.)

this may seem like nitpicking, but it's not -- a MAJOR part of success in gmat math, especially in data sufficiency, comes from establishing exactly which qualities are specified and which quantities aren't.
practicing with sloppily written questions (like this one) can actually undermine the sort of attentive reading that's necessary on the exam.

Hey Vinni,

You're right that he gets there three hours late. However, the method described by Hemant is setting that "three hours later" as right on time. Then, he is calculating what the time would be for "four hours early" from that new benchmark.

If you really wanted to, you could set the original time as t+3 (three hours late), then solve back to t-1 (four hours earlier). It won't actually make any difference, because the four hour difference is the same whether it's t and t+4 or t-1 and t+3.

Does that make more sense?

-t

P.S. I want to second Ron's annoyance with the wording of the question. People post a TON of questions on here that do not simulate the real test. If you ever read a single grammatical error in a question, odds are good it comes from a dangerous source. For example, in this case it says "A man traveling at x speed will reach 3 hours late." A grammatically correct sentence would say "A man traveling at x miles per hour will reach his destination three hours late." A grammatically correct version of the whole question might say:

A man travels from City X to City Y, which is d miles away from City X. He travels in a straight line, and arrives in City Y three hours later than he intended. If the trip took t hours, at what speed should he have travelled if he'd wanted to leave at the same time but arrive an hour earlier than he intended?