A man goes half of a certain distance at 40 km/hr, and the other half 60 km/hr. His average speed for the whole journey is-

V = (2 × 40 × 60)/(40 + 60) = 48kmph

Alternative method:
Average speed is total distance travelled divided by total time taken.

Let the distance travelled is 2x or half of distance is x.

The first half of distance x is travelled with speed of 40km/hr, so time taken is x/40 hr.

Similarly time taken to travel the remaining x km is at speed of 60km/hr, so time taken for this 2nd half of distance is x/60.

Now, the total time taken to travel 2x distance is = x/40 + x/60 hr
= 3x/120 + 2x/120hr
= 5x/120 hr
= x/24 hr.

The average speed of car is 2x/(x/24) or 2 × 24 km/hr or 48km/hr.