Question

A car goes down a certain road at an average speed of $40km/h$ and returns along the same road at an average speed of $60km/h$. Calculate the average speed for the round trip. (in km/h)

• A. $57$
  
• B. $58$
  
• C. $48$
  
• D. $69$
  

Answer 1

Answer: (C)

$48$

For one side trip :
Let the distance covered be $S$.
Speed $v_1=40km/h$
Time taken $t_1=\frac{S}{v}=\frac{S}{40}h$
For return trip :
The distance covered be $S$
Speed $v_2=60km/h$
Time taken $t_2=\frac{S}{v_2}=\frac{S}{60}h$
Total distance covered $=2S$
Total time taken $t_{total}=t_1+t_2=\frac{S}{40}+\frac{S}{60}$
So, average speed $v_{avg}=\frac{2S}{t_{total}}$
$⟹v_{avg}=\frac{2S}{\frac{S}{40}+\frac{S}{60}}=\frac{2\times 40\times 60}{40+60}=48km/h$