How do you write an equation from this word problem?

 Mari and Jen each work 20 hours a week at different jobs. Mari earns twice as much as Jen. Together they ear $480. How much does each girl earn in a week?

 

Let Jen's hourly wage=J, then Mar's hourly wage=2J, but we have:

20J + 40J=480

60J=480 divide both sides by 60,

J=480/60=$8.00 per hour, Jen's hourly wage, and,

$8.00 X 2=$16.00 per hour, Mar's hourly wage, therefore,

$8 X 20=$160 amount that Jen earns in a week, and,

$16 X 20=$320 amount that Mari earns in a week.

but whats the function?

 

♥and thanks♥