If it takes 2 minutes to fill the tank with 3 inches, it would take 4 minutes to fill the tank with 6 inches. From there, it would take 6 minutes to fill the tank with 9 inches, it would take 8 minutes to fill the tank with 12 inches, it would take 10 minutes to fill the tank with 15 inches and finally it would take 12 minutes to fill the tank with 18 inches, which is 10 minutes after the first 2 minutes. He is correct.