Question

A car completes the journey A to B at a constant speed of 20km/hr and covers the return journey from B to A at a constant speed of 30km/hr. The average speed for the total journey from A to B and back is
1. 24km/hr
2. √600km/hr
3. 25km/hr
4. 50km/hr

Answer 1

First we have to find the sum of reciprocals of speeds.
1/20 + 1/30
= (2+3)/60
= 5/60
(By harmonic mean)
Now average speed =
2 ÷ 5/60
= 2 x 60/5
= 2 x 12
= 24 km/hr

Correct option A

Hope This Helps You!

Answer 2

Given that
complete journey speed=20km/h
and return journey speed 30km/h

firstly we have find reciprocal speed

=1/20 + 1/30

we have find lcm 20 and 30

So

=2+3/60
= 5/60

Now we find average speed

we know that

avarage speed=total distance/elasped time

=2/5/60
=2×60/5
=2×12
=24
avarage speed = 24km/h