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?

$\frac{540}{37}$km/h

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