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 [tex] \frac{1}{18}+ \frac{1}{H} = \frac{1}{12} [/tex]. We need a common denominator, which is 36. [tex] \frac{2}{36} + \frac{1}{H} = \frac{3}{36} [/tex]. Doing the simplifying we have [tex] \frac{1}{H}= \frac{1}{36} [/tex] and H = 36 minutes.
