Question

A motorcyclist drove 7 km at 57km/h and then another 7 km at 81 km/h. What was the average speed? ​

Answer 1

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  

$time= \frac{distance}{speed}$

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 =  $14/(7/57+7/81)=(14 \times 57 \times 81)/945=66.9 km/h$

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.