A car travels 100 km at a speed of 60 km/h and returns with a speed of 40 km/h. Find the average speed for the whole journey.

For first journey 

Distance travelled $=100\ km$

Speed $60\ km/h$

Therefore, Time taken for the journey $t_1=\frac{distance}{time}$

$=\frac{100}{60}$

$=\frac{5}{3}\ h$

For Retuning journey:

Distance travelled $=100\ km$

Speed $=40\ m/s$

Therefore, time taken for the returning $t_2=\frac{distance}{time}$

$=\frac{100}{40}$

$=\frac{5}{2}\ h$

Therefore, Average speed of the car $=\frac{Total\ distance}{Total\ time}$

$=\frac{100+100}{\frac{5}{3}+\frac{5}{2}}$

$=\frac{200}{\frac{10+15}{6}}$

$=\frac{200}{\frac{25}{6}}$

$=\frac{200\times6}{25}$

$=48\ km/h$