A $399\text{m}$ long train crosses a pillar in $57\text{seconds}$. Find the speed of the train in $\text{km/hr}$.

1. When a train crosses a stationary pillar, the total distance traveled is equal to the length of the train itself.
$$\text{Distance}=399\text{m}$$

$$\text{Time}=57\text{seconds}$$

2. Calculate the speed in meters per second ($\text{m/s}$):

$$\text{Speed}=\frac{\text{Distance}}{\text{Time}}=\frac{399}{57}=7\text{m/s}$$

3. Convert the calculated speed from $\text{m/s}$ to kilometers per hour ($\text{km/h}$) by multiplying by $\frac{18}{5}$:

$$\text{Speed in km/h}=7\times \frac{18}{5}=\frac{126}{5}=25.2\text{km/h}$$