Your answer is $1620.
To solve this, first look at what we're given. $324 is spent a month on fuel, and fuel is 20% of their income. Therefore, $324 is 20% of the family's income per month.
Now, to solve this, we're looking for 100% of the family's income. 20% is 1/5 of 100%, so therefore $324 is 1/5 of the family's full income.
To get 100%, then multiply 20% by 5 to get 100%, and therefore multiply $324 by 5 to get the full income.
$324 * 5 = 1620, or your answer.
Good luck!
