One third part of a certain journey is covered at the speed of 10 km/hr, one fourth part at the speed of 15 km/hr and the rest part at the speed of 20 km/hr. What will be the average speed for the whole journey?

240/17 km/hr

Let total distance = 120 km

\(\frac{1}{3}\) rd of total distance = \(\frac{1}{3}\) × 120 = 40 km

Time 1 = \(\frac{Distance}{Speed}\)

= \(\frac{40}{10}\) = 4 hours

\(\frac{1}{4}\) th of total distance = \(\frac{1}{4}\) × 120 = 30 km

Time 2 = \(\frac{30}{15}\) = 2 hours

Remaining distance = 120 - 40 - 30 = 50km

Time 3 = \(\frac{50}{20}\) = 2.5 hour

Average Speed = \(\frac{120}{4 + 2 + 2.5}\)

= \(\frac{240}{17}\)  km/h