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?
Answer & explanation
Correct answer: option 1
Let total distance = 200 km
\(\frac{1}{5}\)th of 200 = 40 km
Time 1 = \(\frac{40}{30}\) = \(\frac{4}{3}\) hours
\(\frac{1}{4}\)th of 200= 50 km
Time 2 = \(\frac{50}{25}\) = 2 hours
Remaining distance = 200 - 40 - 50 = 110 km
Time 3 = \(\frac{110}{60}\) = \(\frac{11}{6}\) hours
Total time = \(\frac{4}{3}\) + 2 + \(\frac{11}{6}\) = \(\frac{31}{6}\) hours
Average speed = \(\frac{200 × 6 }{31}\) = 38.7 km/h