Abdul while driving to school computes the average speed for his trip to be 20 km h–1.

Solution:
Let one way distance for his trip be S.

Let $t_{1}$ be the time for his trip from home to school and $t_{2}$ be the time for his return trip.

Then $\mathrm{t}_{1}=\frac{\mathrm{S}}{\mathrm{v}_{1}}=\frac{\mathrm{S}}{20} \mathrm{~h}$, and

$\mathrm{t}_{2}=\frac{\mathrm{S}}{\mathrm{v}_{2}}=\frac{\mathrm{S}}{40} \mathrm{~h}$

Therefore, total time of trip is

$\mathrm{T}=\mathrm{t}_{1}+\mathrm{t}_{2}=\frac{\mathrm{S}}{20}+\frac{\mathrm{S}}{40}=\frac{3 \mathrm{~S}}{40} \mathrm{~h}$

Total distance covered $=2 \mathrm{~S}$

Therefore, average speed of Abdul

$\mathrm{V}_{\mathrm{av}}=\frac{\text { total distance }}{\text { total time }}=\frac{2 \mathrm{~S} \times 40}{3 \mathrm{~S}}=26.6 \mathrm{kmh}^{-1}$