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1. 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 :

A. 10 minutes

B. 15 minutes

C. 20 minutes

D. Data inadequate

Solution:

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{x}{300}

m/sec
Length of the second train = 2x meters
Length of the platform = 2x meters
∴ Required time

\begin{aligned} = [\frac{2 x + 2 x}{(\frac{x}{300})}] sec \\ = (\frac{4 x \times 300}{x}) sec \\ = 1200 sec \\ = \frac{1200}{60} min \\ = 20 minutes \end{aligned}

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