A car travels a certain distance at 24 km/h and comes back at 36 km/h. Find the average speed for total journey.

When a certain distance is covered at speed a and the same distance is covered back at speed b, the average speed for the whole journey is given by the harmonic mean:
Average Speed=
a+b
2ab
​

Given a=24 km/h and b=36 km/h:

Average Speed=
24+36
2×24×36
​

Average Speed=
60
1728
​
=28.8 km/hr