Two railroad tracks are perpendicular to each other. At 12:00 P.M there is a train at each track approaching the crossing at 50 kph, one being 100 km and the other 150 km away from the crossing. How...

Two railroad tracks are perpendicular to each other. At 12:00 P.M there is a train at each track approaching the crossing at 50 kph, one being 100 km and the other 150 km away from the crossing. How fast in kph is the distance between the two trains changing at 4:00 P.M?