In word problems, remember that not every number they give you is important. For example, here, they tell you that they both work 20 hours a week. This information isn't important in creating an equation to represent the situation.
Let's say that x is the amount that Jen makes. Therefore, 2x will be the amount that Mari makes since she makes twice as much as Jen. Together, the amounts they make adds up to 480, meaning that the entire equation will be:
x + 2x = 480
All we need to do is solve this for x, plug x back into the equation, and solve.
1x + 2x = 480 (combine 1x and 2x) 3x = 480 (divide both sides by 3) x = 160
Now, plug 160 in for x in the original equation to check and solve for how much Mari makes (she's represented by 2x).
