Question

A car moves a distance of 200m it cover the first half of the distance at a speed of 40km/h.and the second half of distance at speed v . If the average speed is 48
km/h , then the value of v is

Answer 1

First half of distance = 100m

speed = 40km/h or 100/9 m/s

so time taken= 100÷ 100/9 = 9 second

Given avg speed = 48km/h = 40/3 m/s
Distance= 200m
so total time is 15s

now 9 s has passes covering 100 m. time left = 6s

Distance left = 100m
so speed(v) = 100/6
= 50/3 m/s or 90 km/h

Answer 2

If a person covers equal distances with different speed ,then the average speed is the harmonic mean of the two speed.
So,
Average Speed= 2V1V2/V1+V2 (Harmonic Mean of the speed V1 and V2)

Following this formula we can solve this question as shown in the snap.

Always remember if a body covers equal distances with same velocities then use harmonic mean of the velocities to find average velocity and if a body covers same or different distances in equal intervals of time then use arithmetic mean to find average velocity.

Hope it helps.
If you have any doubt regarding this you can ask it in the comment section.