Discussion
Two trains of equal lengths take 10 seconds and 15 seconds respectively to cross a telegraph post. If the length of each train be 120 metres, in what time (in seconds) will they cross each other travelling in opposite direction?
Answer: B
A telegraph post is a point, so each crossing gives that train's speed.
\[ v_{1} = \frac{120}{10} = 12 \text{ m/s}, \qquad v_{2} = \frac{120}{15} = 8 \text{ m/s} \]
Running opposite ways they close at the sum and must cover both lengths:
\[ t = \frac{120 + 120}{12 + 8} = \frac{240}{20} = 12 \text{ seconds} \]
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