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What is the spead of train Problem Solution

In a Tunnel 1 KM long. Two friends are standing at 600m inside the Tunnel (i.e. 600m from 1 side and 400m from other).Suddenly they heard the whistle of a train.Both started to run on opposite direction at spead of 10 KM/Hr.Both of them just survived.
What is the speed of train?

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Assume the whistle sound doesn't take time to travel.
600m (0.6km) takes 0.6 / 10 = .06 (hr)
400m (0.4km) takes 0.4 / 10 = .04 (hr)
Since both of them "JUST" survived. That means the train enter the tunnel at 0.04 hr and exit tunnel at 0.06 hr. That means it takes the train 0.06 - 0.04 = 0.02 hr to travel 1 km.
Therefore, the speed of the train is 1km / 0.02 hr = 50 km/hr