A jogger is running through a tunnel at a speed of 5 mph. He is 5/8 of the way through the tunnel when he hears a train approaching from behind. If he continues running forward (the remaining 3/8 of the tunnel), the train will reach the tunnel's entrance just as he exits. If he turns around and runs back (the 5/8 of the tunnel), the train will reach the tunnel's entrance just as he returns there. We need to determine the speed of the train.