Question

a train is 360m long is running at a speed of 45 km/hr.What time will it take to cross a 140m long bridge

Answer 1

total distance is 360+140=500m
now 45km/hr
=45×5/18
=12.5m/s
time=distance/speed
=500/12.5
=40sec

Answer 2

$speed \: = 45km {hr}^{ - 1} = 12.5m {s}^{ - 1} \\ total \: distance \: it \: has \: to \: cover \\ = length \: of \: train + length \: of \: bridge \\ = 360 + 140 = 500m \\ \\ we \: know \: that \: \\ speed = \frac{distance}{time} \\ time = \frac{distance}{speed} \\ = \frac{500}{12.5} = 40sec$