(solved)Question 20 SSC-CGL 2020 March 3 Shift 1

Question 20
SSC-CGL 2020 Mar 3 Shift 1

A train crosses a pole in 18 seconds, but a 170 meter long bridge in 36 seconds. What is the speed of the train?

Solution in Short

The train takes twice the time to cover the bridge, so the length of the train and bridge is same. It crosses the pole in 18 seconds, so its speed is (170/18) m/s = 170/18 x 18/5 = 34 kph ans!

Solution in Detail

Let speed = $\displaystyle x$ m/s

When the train crosses the pole, it travels its own length.

So its length = (its speed) x time

$\displaystyle \therefore = x \times 18 = 18x$

When it crosses a bridge, it travels its own length + the length of the bridge.

So $\displaystyle 18x + 170 = \text{speed} \times \text{time}$

$\displaystyle \therefore 18x + 170 = 32x$

$\displaystyle \implies x = \frac{170}{18} $ m/s

$\displaystyle = \frac{170}{18} \times \frac{18}{5} = 34$ kph