Question

A 150 metres long train passes a bridge at rate 90 km/h in 20 seconds. What is the length of the bridge?

Answer 1

$\sf \: Time \: = \: 20 \: seconds$

$\sf \: Speed \: of \: the \: train \: = \: 90 \: km/h \: or \: 25 \: m/s$

$\sf \: Let \: the \: length \: of \: the \: bridge \: be \: x$

$\sf \: Distance \: covered \: = \: Length \: of \: the \: bridge \: + \: Length \: of \: the \: train$

$\sf = \: x \: + \: 150$

$\sf \: \frac{x + 150}{25} = 20\:\:\:\:\: \:\:\:\\ \\ \sf \: x + 150 = 20 \times 25 \\ \\ \sf \: x \: + \: 150 = 500 \: \: \:$

$\sf \: x \: = \: 500 - 150 \\ \\ \sf \: x \: = \: 350 \: \: \: \: \: \: \: \: \: \: \:$

$\color{aqua} \sf \: Hence \: the \: length \: of \: the \: bridge \: is \: 350 \: metres.$