It is said that the smarter a person is, the more difficulty they have in solving
this puzzle. I don't know how much truth there is in that, but it took me about 30 minutes. Dr. Richard Duke at the University of Michigan, who uses this puzzle as a demonstration in his gaming/simulation courses, took more than a year to figure it out initially. Some people solve it in mere seconds.
The page has all the information you need to solve the puzzle. Don't spoil the answer for everybody else if you figure it out. Have fun!