Sammy can paint the house by himself in 15 hours. With Nolan's help, it only takes 9 hours. How long would it take for Nolan to paint the house by himself?
Similar question that will help you:
If it takes Sammy 15 hours to paint the house, that means that he paints 1/15 of the house per hour. If both Sammy and Nolan paint for 9 hours, then Sammy painted 9/15= 3/5 of the house. That means that Nolan painted 2/5 of the house in 9 hours. That means it takes Nolan 9 / 2 * 5= 22.5 hours.