Based on your schedule, having a car will cut your transportation costs enough to squeeze in a 4 hour shift each weeknight at the fast food store paying $9 per hour. you estimate that the car will cost you $100 per week to own and operate. your take-home pay will be 70% of your gross pay. is it worth it to get the car?

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.