@nm oh wow that was super helpful!! thank you very much. 

ErDE:=diff(Er(t),t)=30-Er(t)/500*60;
solEr:=dsolve({ErDE,Er(0)=0},Er(t));
OnDE:=diff(On(t),t)=(rhs(solEr))/500*60-On(t)/1500*60; 
solOn:=dsolve({OnDE,On(0)=0},On(t));
initvals := [[Er(0)=0,On(0)=0], [Er(0)=500,On(0)=0],[Er(0)=500,On(0)=1500]];
DEtools[DEplot]( {ErDE,OnDE} , {Er(t),On(t)}, t=0..150, initvals, numpoints=100, linecolour=[blue,red,yellow]); 

Here is my code.