Explanation:

Length of the train = 100 m
Length of the tunnel = 200 m
Total length = 100 + 200 = 300 m = total distance
And tme taken to cross the tunnel = 21.6 sec
Speed = (100 + 200 /21.6) m/sec.
= (300 /21.6 x 18/5)
= 50 km/hr