If you earn $21 an hour, what is the most you should pay for rent each month?

To answer this question you either need to supply more information.

Although if you week 40 hours a week and at $21 an hour is $840 a week then you multiply that by 4 you get $3360. So if the only bill you are paying for each month is rent, then you can afford $3360 rent payment each month.

Answer is $3360 rent payment each month.