A train moves past a telegraph post and a bridge 240 m long in 8 seconds and 20 seconds respectively. What is the speed (km/h) of the train?

Question: A train moves past a telegraph post and a bridge 240 m long in 8 seconds and 20 seconds respectively. What is the speed (km/h) of the train?

Solution:
Let,
the length of the train = x metres
and its speed = y m/sec

ATQ,
x/y = 8
⇒ x = 8y

and,
(x + 240)/20 = y
⇒ x + 240 = 20y
⇒ 8y + 240 = 20y
⇒ 240 = 20y - 8y
⇒ 12y = 240
∴ y = 20

∴ The speed of the train = 20 m/sec
= 20 × (18/5) km/h
= 72 km/h