$\Delta x=\Delta {{x}_{1}}+\Delta {{x}_{2}}=120+(-50)=70m$ $\Delta x=\Delta {{x}_{1}}+\Delta {{x}_{2}}=15+20=35s$ $\overline{v}=\frac{\Delta x}{\Delta t}=\frac{70}{35}=2\frac{m}{s}$