Question

A car covers $\frac{1}{3}$ part of total distance with a speed of $20km/hr$ and second $\frac{1}{3}$ part with a speed of $30km/hr$ and the last $\frac{1}{3}$ part with a speed of $60km/hr$. The average speed of the car is

• A. $55km/hr$
  
• B. $45km/hr$
  
• C. $30km/hr$
  
• D. $37.3km/hr$
  

Answer 1

The correct option is C $30km/hr$


$\text{Average speed}=\frac{\text{Total distance}}{\text{total time}}$
$v=\frac{d}{t_1+t_2+t_3}$
$v=\frac{d}{\frac{d}{3v_1}+\frac{d}{3v_2}+\frac{d}{3v_3}}$
$v=\frac{3v_1{v}_2{v}_3}{v_2{v}_3+v_3{v}_1+v_1{v}_2}$
Putting values
$v=\frac{3\times 20\times 30\times 60}{1800+1200+600}$
$=30km/hr$
Final Answer: (𝑏)