One-fifth of a journey is covered at a speed of 30 km/h, one-fourth of the journey at a speed of 25 km/h and the rest at 60 km/h. What is the average speed (in km/h, correct to one decimal place) for the whole journey?

Explanation

Given: 1/5 of journey = 30kmph 1/4 of journey = 25kmph (1 - 1/5 - 1/4 = 11/20) of journey = 60kmph Formula: Average speed = Total distance/Total time Calculation: Let total distance of journey be a km. Time taken for 1/5 of journey = a/150 hours Time taken for 1/4 of journey = a/100 hours Time taken for 11/20 of journey = 11a/1200 hours Total time taken for a km = a/150 + a/100 + 11a/1200 = 31a/1200 hours ∴ Average speed = a/(31a/1200) = 38.7kmph