Question 76619
This is a substitution problem once you get the equations. 
The two equations are, when j=jake and l=lisa
3l=j
j-l=400
you substitute the "j" in the second equation the the first equation.
so you gwt 3l-l=400.
then you solve the simple algebraic equation to get l=200.
then to get how many jake sells, you plug in what l equals into either equation.
3(200)=j
j=600