A train $100\ m$ long is moving with the velocity of $72\ km/hr$. Find the time it takes to cross the bridge which is $2\ km$ long?

Here given, length of the train $=100\ m$

Length of the bridge $=2\ km=2\times1000\ m=2000\ m$

Therefore, total distance $=2000\ m+100\ m=2100\ m$

The velocity of the train $v=72\ km/hr=72\times \frac{5}{18}\ m=20\ m/s$

Therefore, time taken to cross the bridge$=\frac{distance}{velocity}$

$=\frac{2100}{20}$

$=105\ sec=1\ minute\ 45\ seconds$

Therefore, the train will take $1\ minute \ 45\ seconds$ to cross the bridge.