In what time will a train 100 metres long cross an electric pole, if its speed is 144km/hr?

Hint: In this problem, we have to find the time at which a train 100 metres long crosses an electric pole with its speed is 144km/hr. Here we can use the formula \[Time=\dfrac{\text{Distance}}{speed}\]. Here we have distance 100 metres, we can now convert the given unit of the speed from km per hour to m per sec. we can then substitute the result in the formula to get the time.

Complete step by step answer:
Here, we have to find the time at which a train 100 metres long crosses an electric pole with its speed is 144km/hr.
We can first convert the given speed from km per hour to m per sec as the given distance is in metres.
We know that, 1 km = 1000 m and 1 hour = 60 hours and 1 hour = 60 seconds, we can now convert them, we get
\[\Rightarrow Speed=144\times \dfrac{1000}{60\times 60}=144\times \dfrac{5}{18}=40m/\sec \]
We know that to find the time we can use the formula \[Time=\dfrac{\text{Distance}}{speed}\], where distance = 100m and speed = 40m/sec, we get
\[\Rightarrow Time=\dfrac{100}{40}=2.5s\]
Therefore, the required time is 2.5 seconds.

Note: We should always remember that if we have different unit measurements in the same question, then we have to convert them into one unit, as here we have distance in meters, so we have converted the speed from km/hr to m/sec. We should also remember the formula \[Time=\dfrac{\text{Distance}}{speed}\].