Question

A car travelling with $\frac{5}{7}$ of its usual speed covers $42$ km in $1$ hour $40$ min $48$ sec. What is the usual speed of the car?

• A. $17\frac{6}{7}$ km/hr
• B. $25$ km/hr
• C. $30$ km/hr
• D. $35$ km/hr

Answer 1

The correct option is D $35$ km/hr

Let the usual speed of the car be x km/hr.

Time = 1 hour 40 min 48 sec $=1+\frac{40}{60}+\frac{48}{3600}hrs$

$=1+\frac{2}{3}+\frac{1}{75}hrs$

$=\frac{75+50+1}{75}hrs$

$=\frac{126}{75}hrs$

Given, $\frac{5}{7}x\times \frac{126}{75}=42\Rightarrow x=\frac{42\times 7\times 75}{5\times 126}$

$=35km/hr$