The system of equations that represents this situation is:
[tex]t+w=70[/tex]
[tex]20+w=t[/tex]
The first equation is "time spent on the treadmill (t) plus time spent lifting weights equals total time spent working out at the gym (70)". The second equation illustrates that the time spent on the treadmill was 20 minutes longer than the time lifting weights.
I hope this explanation makes sense!