A travels a certain distance with three different speeds; first 3 km with a speed of 9 km/h, second 6 km with a speed of 15 km/h, and third 9 km with a speed of 18 km/h. What is the average speed of A for the whole journey?
Answer & explanation
Correct answer: option 4
Average speed = \(\frac{Total\; DIstance}{ Total \;Time}\)
Total distance = 3 + 6 + 9 = 18 km
Total time = \(\frac{3}{9}\) + \(\frac{6}{15}\) + \(\frac{9}{18}\)
= \(\frac{30 + 36 + 45}{90}\)
= \(\frac{111}{90}\) = \(\frac{37}{30}\)
Average speed = \(\frac{18 × 30}{37}\)
= \(\frac{540}{37}\) km/h