What we're looking for is if the extra take-home pay will cover the extra $100/week in expenses.
If you work a 4 hour shift each weeknight, then multiplying that by the number of weeknights:
4 x 5 = 20
we find that you will work 20 extra hours per week.
Since you are making $9/hour, multiplying that by how many hours you will be working we can find how much extra your gross pay will be per week:
9 x 20 = 180
You will be making $180 extra gross pay per week.
Since you take home 70% of your gross pay, if we multiply your gross pay by 70$, we will find your take-home pay:
180 x 0.70 = 126
Your extra take-home pay will be $126 dollars.
Since $126 is greater than $100, it is worth it to get the car.
