You can write this as two simultaneous equations:
[tex]L = 8 + T[/tex]
and
[tex]L = 2T[/tex]
where L is the number of miles Lisa walked and T is the number of miles Tim walked. Since you have two equations for L, you can set the right hand sides equal to each other like so:
[tex]2T = T+8[/tex]
and then solve for T to get the number of miles Tim walked. Then just plug in what you get for T into either of the two original equations to get L.
Additionally, if you plot the two original linear equations, you can see that the solution is the point where the two graphs cross.
