Delhi Police Constable Exam Model Paper 4

Given,
Car covers 10 km distance with the speed of = 5 km/hr
Car covers 20 km distance with the speed of = 10 km/hr
Formula :Average speed = Total time/total distance
Calculation:
Time taken by car to cover 10 km distance with the speed of 5 km/hr = 10/5 = 2 hr
Time taken by car to cover 20 km distance with the speed of 10 km/hr = 20/10 = 2 hr
Total distance = 10 + 20 = 30 km
Total time = 2 + 2 = 4 hrs
∴ Average speed = 30/4 = 7.5 km/hour
Mistake Point:
As the distance is different therefore we will not use the below formula.
Average Speed $=\frac{\left(2\times \mathit{S}1\times \mathit{S}2\right)}{\left(\mathit{S}1+\mathit{S}2\right)}$