One-fourth of certain journey is covered at the rate of 25 km/h, one-third at the rate of 30 km/h and the rest at 50 km/h. Find the average speed for the whole journey.

Answer: Option C

Let distance be 120 km Hence 30 km is covered by @25 kmph and 40 km covered by @30 kmph and rest 50 km has been covered @50 kmph
Now,
$$\begin{array}{rl}& \text{average}=\frac{120}{\text{total}\text{time}\text{taken}}\\ & =\frac{120}{\frac{30}{25}+\frac{40}{30}+\frac{50}{50}}\\ & =\frac{3600}{106}\\ & =\frac{1800}{53}\text{km/h}\end{array}$$