Train puzzle

Two boys walking in the woods decide to take a shortcut through a railroad tunel. When they had walked 2/3 of the way, their worst fears were realized. A train was coming in the opposite direction, nearing the tunnel entrance. They boys panicked and each ran for a different end of the tunnel. Both boys ran at the same speed, 10 miles per hour Each boy escaped from the tunnel just at the instant the train would have squashed him. Assuming the train's speed was constant, and both boys were capable of instantaneous reaction and acceleration, how fast was the train going?

Answer:

When the first boy just escapes at the end of tunnel, the second boy is 1/3rd away from entrance, and when the train reaches the entrance, the second boy just escapes, so when second boy travells x, the train travells 3x, i.e speed is 10 mph