Pawan travels from X to Y at the speed of 10 km/h, from Y to Z at 15 km/h and from Z to W at 20 km/h. If XY = YZ = ZW (in km), then the average speed of Pawan for the whole journey is:

A

$$13\frac{10}{13}$$ km/h

B

$$13\frac{11}{13}$$ km/h

C

$$13\frac{8}{13}$$ km/h

D

$$13\frac{9}{13}$$ km/h