Answer:
The average speed of the car is 66.9 km/h
Explanation:
Here distance covered with the speed 57 km/h=7 km
distance covered with the speed of 81 km/h=7 km
Average speed is equal to the ratio of total distance to the total time.
total distance= 7 + 7= 14 km
[tex]time= \frac{distance}{speed}[/tex]
time taken to cover the first 7 km= 7/57 h
time taken to cover the second part of the journey = 7/81 h
average speed = [tex]14/(7/57+7/81)=(14 \times 57 \times 81)/945=66.9 km/h[/tex]
Shortcut:
When equal distances are covered with different speeds average speed=2 ab/(a+b) where a and b are the variable speeds in the phases.
