A bus travels the first one-third distance at a speed of $20 \mathrm{~km} / \mathrm{h}$, the next on

Question

A bus travels the first one-third distance at a speed of $20 \mathrm{~km} / \mathrm{h}$, the next one-third at $30 \mathrm{~km} / \mathrm{h}$ and the last one-third at $60 \mathrm{~km} / \mathrm{h}$. The average speed of the bus is

Professor PC Thomas Classes · Answer

(a) : Total distance travelled $=s$ \ Total time taken $=\frac{s / 3}{20}+\frac{s / 3}{30}+\frac{s / 3}{60}$ \ $=\frac{s}{60}+\frac{s}{90}+\frac{s}{180}=\frac{3s+2s+s}{180}=\frac{6s}{180}=\frac{s}{30}$ \ Average speed $=\frac{	ext{total distance travelled}}{	ext{total time taken}}=\frac{s}{s / 30}=30 \mathrm{~km} / \mathrm{h}$