[tex]Formula\ for\ velocity\\\\ v=\frac{s}{t}\\\\s-distance\\\\ t-time\\\\
t=20min\\\\ v_1=70m/min\\\\v_2=210m/min\\\\s_1- distance\ when\ he\ walk \\\\s_2=distance\ when\ he\ run\\\\
s_1=1800-s_2\\\\
[/tex][tex] \left \{ {{70=\frac{s_1}{t_1}\ \ |*t_1} \atop {210=\frac{1800-s_1}{20-t_1}\ \ |*(20-t_1)} \right. \\\\
\left\{{70*t_1=s_1}\atop{210(20-t_1)=1800-s_1}}\right\\\\
\left \{ {{70t_1=s_1} \atop {4200-210t_1=1800-s_1}} \right.\\\\ \left \{ {{70t_1=s_1} \atop {4200-1800=210t_1-s_1}} \right.\\\\ \left \{ {{70t_1=s_1} \atop {2400=210t_1-s_1}} \right.\\\\Substituting\ method:\\\\
2400=210t_1-70t_1\\\\
2400=140t_1\ \ |:140 t_1=\frac{2400}{140}=\frac{240}{14}=\frac{120}{7}=17\frac{1}{7}min\\\\t_2=2\frac{6}{7}min[/tex][tex]s_2=v_2*t_2\\\\
s_2=210*2\frac{6}{7}\\\\
s_2=210*\frac{20}{7}=600m\\\\
He\ ran\ 600m\ or\ more.[/tex]
