Question

A car moves through $20\text{km}$ with a speed of $40\text{km/hr}$ and the next $20\text{km}$ with a speed of $60\text{km/hr}$. Calculate its average speed.

• A. $45\text{km/hr}$
• B. $48\text{km/hr}$
• C. $52\text{km/hr}$
• D. $58\text{km/hr}$

Answer 1

Answer: (B)

Step 1: Given that:

The speed of car for the first $20km$ = $40km{h}^{-1}$

The speed of the car in next $20km$ = $60km{h}^{-1}$

Step 2: Calculation of average speed of the car:

Average speed= $\frac{Totaldistancecoveredbythebody}{Totaltimetakenbythebody}$

Let the time taken by the car to cover the first $20km$ be $t_1$ and the time taken by the car to cover the next $20km$ is $t_2$, then

Using formula, Time= $\frac{Distance}{speed}$ ,

$t_1=\frac{20km}{40km{h}^{-1}}$

$t_1=\frac{1}{2}hr$

And,

$t_2=\frac{20km}{60km{h}^{-1}}$

$t_2=\frac{1}{3}hr$

Thus,

The average speed of the car will be;

$v_{avg}=\frac{20km+20km}{\frac{1}{2}hr+\frac{1}{3}hr}$

$v_{avg}=\frac{40}{\frac{3+2}{6}}$

$v_{avg}=\frac{40}{\frac{5}{6}}$

$v_{avg}=\frac{40}{5}\times 6$

$v_{avg}=8\times 6$

$v_{avg}=48km{h}^{-1}$

Thus,

The average speed of the car will be $48km{h}^{-1}$.