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1. Two trains are coming from opposite directions with speed of 75 km/hr and 100 km/hr on to parallel tracks. At some moment the distance between them is 100km. After T hours, distance between them is again 100 km. T is equal to?

A. 1 hr

B. $1 \frac{1}{7}$ hr

C. $1 \frac{1}{2}$ hr

D. 2 hr

Solution:

\begin{aligned} Relative speed \\ =  (75 + 100)km/hr \\ =  175 km/hr \\ Time taken to cover 175 km \\ at relative speed  =  1 hr \\ ∴ T  =  Time taken to cover 200 km \\ = (\frac{1}{175} \times 200) hr \\ = \frac{8}{7} hr \\ = 1 \frac{1}{7} hr \end{aligned}

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