What is my average speed?

One approach here, is to assume the distance from $A$ to $B$ to be $40$ km. So you travelled from $A$ to $B$ in one hour. The ratio of distance $\frac{AB}{BC}$ $=$ $\frac{1}{2}$ , we can see that $BC=80$ km. From here, solve for the time travelled from $B$ to $C$ and that is $\frac{4}{3}$ $hours$ . Now solve for the average speed in the whole trip.

$average$ $speed=$ $\frac{total distance}{total time}=$ $\frac{40+80}{1+4/3}=51.43kph$