He is correct. You can explain this because you know it takes him 2 minutes to fill it 3 inches. Since the tub is 18 inches deep, you can do 18 inches ÷ by 3 inches. This will give you 6. Remember that it takes 2 minutes to fill it 3 inches, so you should multiply 2 minutes by 6. It will take him approximately 12 minutes to fill it all the way.