A $280\text{ m}$ long train is running at $108\text{ km/hr}$. In how much time will it cross a bridge of length $170\text{ m}$?

First, let's convert the speed from $\text{km/hr}$ to $\text{m/s}$:
$$\text{Speed}=108\times \frac{5}{18}=6\times 5=30\text{ m/s}$$
The total distance to be covered to cross the bridge is the sum of the lengths of the train and the bridge:
$$\text{Total Distance}=280+170=450\text{ m}$$
Time taken:
$$\text{Time}=\frac{\text{Distance}}{\text{Speed}}=\frac{450}{30}=15\text{ seconds}$$