A train takes 5 minutes to cross a telegraphic post. Then the time taken by another train whose length is just double of the first train and moving with same speed to cross a platform of its own length is :

Answer: Option C

Let the length of the train be x metres.
Time taken to cover x meters = 5 min
= (5 × 60) sec
= 300 sec
Speed of the train = $$\frac{\text{x}}{300}$$ m/sec
Length of the second train = 2x meters
Length of the platform = 2x meters
∴ Required time
$$\begin{array}{rl}& =\left[\frac{2\text{x}+2\text{x}}{\left(\frac{\text{x}}{300}\right)}\right]\text{sec}\\ & =\left(\frac{4\text{x}\times 300}{\text{x}}\right)\text{sec}\\ & =1200\text{sec}\\ & =\frac{1200}{60}\text{min}\\ & =20\text{minutes}\end{array}$$