Question

One car moving on a straight road covers one-third of the distance with $20km/hr$ and the rest with $60km/hr$. The average speed is

• A. $36km/hr$
  
• B. $80km/hr$
  
• C. $40km/hr$
  
• D. $46\frac{2}{3}km/hr$
  

Answer 1

The correct option is A $36km/hr$


Given,
$v_1=20km/hr$
$v_2=60km/hr$
Let $t_1$ is the time taken by initial $\left(\frac{1}{3}\right)rd$ distance and $t_2$ for remaining $\left(\frac{2}{3}\right)rd$ distance.

$\text{Average speed}=\frac{\text{Total distance}}{\text{total time}}$
$v_{avg}=\frac{x}{t_1+t_2}$
$v_{avg}=\frac{x}{\frac{x}{3v_1}+\frac{2x}{3v_2}}=\frac{3v_1{v}_2}{2v_1+v_2}$
$v_{a}vg=\frac{3\times 20\times 60}{2\times 20+60}$
$=36km/hr$
Final Answer: (𝐃)