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1. A train moves with a speed of 30 kmph for 12 minutes and for next 8 minutes at a speed of 45 kmph. Find the average speed of the train?

A. 37.50 kmph

B. 36 kmph

C. 48 kmph

D. 30 kmph

Solution:

\begin{aligned} Distance  =  Speed \times Time \\ Distance covered by train with the \\ speed of 30 kmph in 12 minutes is \\ =  30 \times \frac{12}{60} = 6 km \\ Distance covered by the same train \\ with the speed of 45 kmph in 8 minutes is \\ =  45 \times \frac{8}{60} = 6 km \\ Average speed \\ = \frac{total distance}{total time} . \\ \Rightarrow \frac{(6 + 6) km}{(12 + 8) min} = \frac{12}{20} \times 60 \\ =  36 kmph \end{aligned}

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