${{v}_{av}}=\frac{\Delta x}{\Delta t}=\frac{{{v}_{1}}\Delta {{t}_{1}}+0+{{v}_{3}}\Delta {{t}_{3}}}{\Delta {{t}_{1}}+\Delta {{t}_{2}}+\Delta {{t}_{3}}}=\frac{20\times 45\times 60+40\times 27\times 60}{(45+5+40)\times 60}=22\frac{m}{s}$ $22\frac{m}{s}\times \frac{1km}{{{10}^{3}}m}\times \frac{3600s}{1h}=79/2\frac{km}{h}$