Madhya Pradesh Police Exam Model Paper 3

Given:
Speed by car = 30 km/h
Speed by walk = 10 km/h
Formula used:
If equal distance is travel at two different speed, then calculating average speed is as follows:
Average speed = 2(Product of speeds)/(Sum of speeds)
Calculation:
Average speed $=\left(2\times 30\times 10\right)/\left(30+10\right)$
$\Rightarrow 600/40$
⇒ 15 km/h
Alternative method
Let the distance travel in each speed is 30 km.
Average speed = Total distance travelled/Total time taken
$\Rightarrow \left(30+30\right)/\left[\left(30/30\right)+\left(30/10\right)\right]$
$\Rightarrow 60/4$
⇒ 15 km/h