Question Statement :

Two super-fast trains, which travel at speeds of 120 km/hr and 150 km/hr, left the Beijing Railway Station at exactly the same time, and were headed in the same direction. Thirty minutes later, another super-fast train left Beijing Station in the same direction as the previous two. It crossed the faster train an hour and a half after it crossed the slower one. Find the third train’s speed(in km/hr). (Note: Assume all trains to be of negligible length.) 

Type in your answer as an integer. For example, if your answer is 17, type 17