Uniform Circular Motion

A motorcyclist drives from $A$ to $B$ with a uniform speed of $30 \mathrm{km} {\mathrm{h}}^{-1}$ and returns back with a speed of $20 \mathrm{km} {\mathrm{h}}^{-1}$. Find its average speed.

Suppose a boy is enjoying a ride on a merry-go-round which is moving with a constant speed of $10 \raisebox{1ex}{$\mathrm{m}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{s}$}\right.$. It implies that the boy is