A 350 m long train passes over a 250 m long bridge at a speed of 54 km/h. How long will it take to cross the bridge?
a) 40 minutes
b) 2/3 second
c) 2/3 hr
d) 1/90 hr

Hint: Take a careful look at the size of the train and the bridge. The train is 100m longer than the bridge. Therefore, the total distance that the train needs to cover at the beginning is the sum of the length of the train and the bridge. This means that the total distance the train needs to travel is 350m + 250m = 600m. So, by using the formula: $s=\dfrac{d}{t}$ , find the time taken by train to cross the bridge.

Complete step by step answer:
We have:
Length of train = 350 m
Length of bridge = 250 m
Speed of train = 54 km/h
Therefore, $s=\dfrac{54\times 1000}{60\times 60}=15m/s$
As we have, total distance to be covered d = 600 m
So, by using the formula: $s=\dfrac{d}{t}$, we have:
$\begin{align}
  & \Rightarrow 15=\dfrac{600}{t} \\
 & \Rightarrow t=\dfrac{600}{15} \\
 & \Rightarrow t=40\sec \\
 & \Rightarrow t=\dfrac{40}{60\times 60}hr \\
 & \Rightarrow t=\dfrac{1}{90}hr \\
\end{align}$

So, the correct answer is “Option D”.

Note:
The most important thing to look here is the total distance that the train needs to cover to cross the bridge completely.
After this the simple formula of speed is used to calculate the time.