[tex]d-distance\\t-time\\v-speed[/tex]
[tex]d_1=1mile\\d_2=2.5miles\\t_1=t_2\\v_1+6=v_2\\\\v=\frac{d}{t}\to vt=d\to t=\frac{d}{v}\\\\\frac{d_1}{v_1}=\frac{d_2}{v_2}\\\\\frac{1}{v_1}=\frac{2.5}{v_1+6}\\\\cross\ multiply:\\2.5v_1=v_1+6\\2.5v_1-v_1=6\\1.5v_1=6\ \ \ \ \ /:1.5\\v_1=4\\\\Answer:Marcus'\ walking\ speed\ is\ 4\ mph.[/tex]
