${{v}_{av}}=\frac{\Delta {{x}_{1}}+\Delta {{x}_{2}}+\Delta {{x}_{3}}}{\Delta {{t}_{1}}+\Delta {{t}_{2}}+\Delta {{t}_{3}}}=\frac{{{v}_{1}}\Delta {{t}_{1}}+{{v}_{2}}\Delta {{t}_{2}}+{{v}_{3}}\Delta {{t}_{3}}}{\Delta {{t}_{1}}+\Delta {{t}_{2}}+\Delta {{t}_{3}}}$ $\Rightarrow {{v}_{av}}=\frac{(10\times 60)+(25\times 20)+((-5)\times 5)}{60+20+5}=\frac{1075}{85}=\frac{215}{17}\frac{m}{s}$