${{v}_{1}}=0,{{v}_{2}}=120\frac{km}{h},\Delta t=\frac{12}{3600}h$ $\Delta x=\frac{{{v}_{1}}+{{v}_{2}}}{2}.\Delta t=\frac{0+120}{2}\times \frac{12}{3600}=\frac{12\times 60}{3600}=\frac{2\times 10}{100}=0/2km=200m$