Question

A racing car covered 2 rounds of track in 5 minutes, the next 4 rounds in 8 minutes and the last 4 rounds in 7 minutes. Find the average speed of the car. (Length of the track is 4 km)

• A. 100 $\frac{km}{h}$
• B. 120 $\frac{km}{h}$
• C. 140 $\frac{km}{h}$
• D. 160 $\frac{km}{h}$

Answer 1

The correct option is B

120 $\frac{km}{h}$

$Averagespeed$ = $\frac{\text{Total distance}}{\text{Total time taken}}$

Now distance covered in first 2 rounds = 2 $\times$ 4 km

= 8 km

Distance covered 4 rounds = 4 $\times$ 4 km

= 16 km

Total distance covered = 2 rounds + 4 rounds + 4 rounds

= 8 km + 16 km + 16 km

= 40 km

Total time taken = 5 minutes + 8 minutes + 7 minutes

= 20 minutes

= $\frac{20}{60}$ hours ($\because$ 1 minute = $\frac{1}{60}$ hours)

= $\frac{1}{3}$ hours

$\therefore$ Average speed of car = $\frac{40}{\frac{1}{3}}$