A passenger train consists of 18 bogies, each bogie being 15 m long. The train crosses an electric post in 15 sec. Due to some problem, three bogies were detached. The train now crosses the electric post in_______ sec.

Correct option is C

Given:
Total bogies = 18
Length of each bogie = 15 m
Time to cross the electric post = 15 sec
Formula Used:
Speed = (Length of train)/Time
Solution:
Length of train = 18 × 15 = 270 m
=> Speed = $$\frac {270}{15} = 18 m/sec$$​​
Remaining bogies = 18 - 3 = 15
Length of train after 3 bogies have detached = 15 × 15 = 225 m
​$$\text{Speed} = \frac{\text{Length of Train}}{\text{Time}} \\[10pt]\\\Rightarrow 18 = \frac{225}{\text{Time}} \\[10pt]\\\Rightarrow \text{Time} = \frac{225}{18} = 12.5 \, \text{sec}\\$$​​