So you could automatically get rid of B and D since 19 and 17 are not 18.
After that, you have it down to A and C. The way you would find how much she charges for one hour is dividing 720 by 18. When you do that it means you are divding 720$ into 18 different groups each group representing an hour. When you divide 720 by 18 you should get 40. So every hour she makes 40 dollars. 40x 2 hours is 80. I hope I helped you!
