Question

Let a car travel at a speed of 60 km/hr along a
straight road from a point P to another point a
and immediately returns to the point P at a
speed of 90 km/hr. Then for the whole journey,
average speed of the car is
(1) 75 km/hr
(2) 84 km/hr
(3) 66 km/hr
(4) 72 km/hr

Answer 1

Answer:

average speed = 72 km/hr

Answer 2

Answer:

Let a car travel at a speed of 60 km/h along the straight road from a point P to another point Q  and immediately returns to the point P at a  speed of 90km/hr. Then for the whole journey average speed of the car is $72km/h$

Explanation:

The average speed of a body is the total distance divided by the time taken to complete the distance

       average speed= total distance /total time

When a body travels from point P to Q with speed $x km/h$ and returns to P with a speed of $ykm/h$ here the distance covered is the same so the average speed is

$v=\frac{2xy}{x+y}$

from the question,

speed of the car from P to Q is $x=60km/h$

speed of the car from Q to P is $y=90km/h$

therefore the average speed is $v=\frac{2*60*90}{60+90}=72km/h$