Question

A boy takes $2$ hours to reach his school from the house which is $3km$ away from it and takes $90$ minutes to come back from school to his house. Calculate the average speed of the boy.

• A. $\text{Average speed}=11m{s}^{-1}$
• B. $\text{Average speed}=31m{s}^{-1}$
• C. $\text{Average speed}=\frac{21}{10}m{s}^{-1}$
• D. $\text{Average speed}=\frac{10}{21}m{s}^{-1}$

Answer 1

The correct option is D $\text{Average speed}=\frac{10}{21}m{s}^{-1}$

Given:
Total time taken by the boy,
$T=2hr+90minutes$
We know that, $1hr=60minutes$
$\therefore$$T=120minutes+90minutes$ $=210minutes$
Total distance travelled by the boy,
$D=3km+3km=6km$
We know that,
$\text{Average speed}=\frac{\text{Total distance covered}}{\text{Total time taken}}=\frac{6km}{210minutes}$
[$1km=1000m,1minute=60s$]
$\Rightarrow$ $\text{Average speed}=\frac{6\times 1000m}{210\times 60s}$$=\frac{10}{21}m{s}^{-1}$.