Answer
$20n$
Work Step by Step
Every week, you gain 20 more dollars.
At week 0, you have 0 dollars.
At week 1, you have 20 dollars.
At week 2, you have 40 dollars.
We notice that the dollar amount is always 20 times more than the week, therefore the formula is $20n$, where $n =$ number of weeks.
