A man travels equal distances of his journey at 40, 30 and 15 km/h. respectively. Find his average speed for whole journey.

Answer: Option A

Required average speed,
$$=\frac{\left(3\times 40\times 30\times 15\right)}{\left(40\times 30\right)+\left(40\times 15\right)+\left(30\times 15\right)}$$
$$=24\text{km/hr}$$

Alternatively
Time taken to traveled $$\frac{1}{3}$$ distance of journey with speed 40 kmph,
$$=\frac{\frac{1}{3}}{40}=\frac{1}{120}$$
Time taken to traveled $$\frac{1}{3}$$ distance of journey with speed 30 kmph,
$$=\frac{\frac{1}{3}}{30}=\frac{1}{90}$$
Time taken to traveled $$\frac{1}{3}$$ distance of journey with speed 15 kmph,
$$\begin{array}{rl}& =\frac{\frac{1}{3}}{15}=\frac{1}{45}\\ & \text{Total time taken}\\ & =\frac{1}{120}+\frac{1}{90}+\frac{1}{45}\\ & =\frac{45}{1080}\\ & \text{Average speed}\\ & =\frac{\text{Total distance traveled}}{\text{Total time taken}}\\ & =\frac{1}{\frac{45}{1080}}\\ & =24\text{kmph}\end{array}$$