The average velocity 

[LATEX]<\overrightarrow{v}>=\frac{\overrightarrow{r}}{t}[/LATEX]

where [TEX]\overrightarrow{r}[/TEX] is the total displacement, t - the total time.

Consider triangle ABC

[LATEX]|\overrightarrow{r}|=|\overrightarrow{AC}|[/LATEX]

[LATEX]|AB| = |BC| \equiv s[/LATEX]

Then

[LATEX]|\overrightarrow{r}|^2=s^2+s^2-2s^2\cdot cos 150^o = 2s^2(1+cos 30^o)=s^2(2+\sqrt{3})[/LATEX]  

The absolute value of the average velocity

[LATEX]|<\overrightarrow{v}>|=\frac{|\overrightarrow{r}|}{t}=\frac{s \sqrt{2+\sqrt{3}}}{\frac{s}{72}+\frac{s}{36}}=\frac{\sqrt{2+\sqrt{3}}}{\frac{1}{72}+\frac{1}{36}}=24\left ( \sqrt{2+\sqrt{3}} \right )\approx 46.36 ~ km/h[/LATEX]

The direction is 60 - 15 = 45 degrees counterclockwise from OX