Acar travelled 100 km with half the distance at 40 km/h and the other half at 80 km/h. find the average speed of the car for the whole journey.

We must look at this question in steps
The first half of the journey is travelled at 40 km/h
Half of 100km is 50 km
Using the formula
Distance = Speed x Time
Speed = Distance / Time
Time = Distance / Speed
We can work out the time:

50km / 40km/h = 1.25 hours

Next we look at the second half of the journey
50km at 80km/h
50km / 80km/h = 0.625 hours

Add together both times to work out how long the entire journey took
1.25 + 0.625 = 1.875 hours

Using the Speed formula from before
Speed = 100km / 1.875 =
53 1/3 km/h or 53.3 recurring km/h