Excited Dog (Solution)
Thinking about the path of the dog makes this problem hard. Instead, think about the path of the people. Combined, Bob and Sam are closing the 2 mile gap between them at a rate of 10 MPH. So that takes 1/5 of an hour.
The dog runs at 25MPH, so how far can he run in 1/5 of an hour? Easy, 5 miles.