You can rent time on computers at the local copy center for a $6 setup charge and an additional $5.50 for every 10 minutes. How much time can be rented for $21 ?

21$ is the start.

You need to take 6$ off because of the setup charge.

21-6 = 15$ left

Every 10 minutes costs $5.50 so you need to divide 5.50 from 15 and multiply that answer by 10. 20 minutes can be rented. with 4$ left