Atrain passes by an observer in 6 seconds and over a bridge 350m long in 20 seconds. find the speed and length of the train.

Let x be the length of the train.

On the basis of the observer;
Speed of the train = x/6

On the basis of the bridge;
Total distance covered by any point of the train= 350+x
Speed = (350+x)/20

Equating the two expressions of speed;
x/6 = (350+x)/20
20(x) = 6(350+x)
20x = 2100+6x
(20-6)x = 2100
14x = 2100
x= 2100/14 = 150 m
Speed = x/6 = (350+x)/20 = 150/6 = 500/20 = 25 m/s.

Therefore,
Length of train = 150 m
Speed of train = 25 m/s

With a continuous thought process.