Fizz×6

If total distance=x

Time taken to cover 1st half of the distance@40 km/h=(x/2)/40

Time taken to cover 2nd half of the distance=(x/2)/60

Average Speed=Total distance/Total time=x/[((x/2)/40)+((x/2)/60)] =48

I supposed that the car must go $240km$ all the way, so each halves are $120km$ . The first half takes $3h$ , the second half takes $2h$ . The average speed is $\frac{distance}{time}$ , therefore it is $\frac{240}{5}=48$

Say that the car covers 120 km. This means that it will take 2.5 hours. Thus, avg. speed=120/2.5=48

First half x/2 km at 40 km/h and other x/2 at 60 gives us time to cover first half x/80 and second half x/120 total time x/48 and total distance is x so we get av. Speed 48..:-)