Rohit takes 30 minutes to walk the 4 kilometres to his school from his house. When he arrives, he discovers that the school is closed, so he returns home in his friend's car which takes 10 minutes. Find his average speed for the whole journey.

Average speed is the total distance covered in the given time.

Average speed =$\frac{\mathrm{Total} \mathrm{distance} \mathrm{covered}}{\mathrm{Total} \mathrm{time} \mathrm{taken}}$

Total distance=  4 + 4 = 8 km
Total time taken = 30 + 10 = 40 min = 40 $÷$ 60  hr

$\therefore$Average speed =$\frac{8}{{\displaystyle \frac{40}{60}}}= \frac{8\times 60}{40}=12 \mathrm{km}/\mathrm{h}$

Therefore, Rohit's average speed will be 12 km/h.