A bus travels a distance of 120 km with a speed of 40 km/h and returns with a speed of 30 km/h. Calculate the average speed for the entire journey.

When bus travels with the speed of $40\ km/h$

$distance_1=120\ km$

$speed_1=40\ km/h$

Therefore, $time_1=\frac{distance_1}{speed_1}$

$=\frac{120\ km}{40\ h}$

$=3\ h$

When the bus returns:

$distance_2=120\ km$

$speed_2=30\ km/h$

Therefore, $time_2=\frac{distance_2}{speed_2}$

$=\frac{120\ km}{30\ km/h}$

$=4\ h$

Therefore, $total\ distance=distance_1+distance_2$

$=120\ km+120\ km$

$=240\ km$

$total\ time=time_1+time_2$

$=3\ h+4\ h$

$=7\ h$

Therefore, the average speed$=\frac{total\ distance}{total\ time}$

$=\frac{240\ km}{7\ h}$

$=34.28\ km/h$

Thus the average speed of the bus is $34.28\ km/h$.