A car travelling with $\frac{5}{7}$ of its actual speed covers 42 km in 1 hr 40 min 48 sec. Find the actual speed of the car.

A. $17 \frac{6}{7}$ km/hr

B. 25 km/hr

C. 30 km/hr

D. 35 km/hr

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Correct Answer : D.

Time taken = 1 hr 40 min 48 sec =

1 h r 40 \frac{4}{5} m i n = 1 \frac{51}{75} h r s = \frac{126}{75} h r s .

Let the actual speed be x km/hr.
Then,

\frac{5}{7} x \times \frac{126}{75} = 42

\Rightarrow x = (\frac{42 \times 7 \times 75}{5 \times 126}) = 35 k m / h r

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