A car travelling with 5/7 of its actual speed covers 42 km in 1 hr 40 min 48 sec. Find the actual speed of the car.

Answer: Option D

Time taken :
$$\begin{array}{rl}& =1\text{hr}40\text{min}48\text{sec}\\ & =1\text{hr}40\frac{4}{5}\text{min}\\ & =1\frac{51}{75}\text{hrs}\\ & =\frac{126}{75}\text{hrs}\end{array}$$
Let the actual speed be x km/hr
Then,
$$\begin{array}{rl}& \frac{5}{7}x\times \frac{126}{75}=42\\ & \text{Or }x=\left(\frac{42\times 7\times 75}{5\times 126}\right)\\ & x=35\text{ km/hr}\end{array}$$