If it takes 12 minutes to fill the tub using H and C, you can get 1/12 of the tub filled in 1 minute. If you use just C it takes 18 minutes to fill it, and you can get 1/18 of the tub filled in 1 minute. If you are looking to see how long it will take to fill the tub with just H, you can get 1/H of the tub filled in 1 minute. H is our unknown. Adding them together takes 12 minutes, so the equation we have is

. We need a common denominator, which is 36.

. Doing the simplifying we have

and H = 36 minutes.