A man is standing on a bridge which is 180 m long. He finds that a train crosses the bridge in 20 seconds but himself in 8 seconds. What is the speed of the train?

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Firts trains crosses the person within 8 seconds. So the equation of speed of train can be written as,
v=x/8   ......(1)
Here, x is the length of the train.

And the train crosses the bridge of 180m within 20 seconds. So the equation of the speed of train can also be written as, 
v=(x + 180)/20   ......(2)

Equalise the equation (1) and (2).
20x=8x+1440
12x =1440
x=120 m

Substitute 120 m for x in the equation (1).
v=15 m/s
Thus, the speed of the train is 15 m/s.

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