math algebra

when only the cold water valve is opened, a washtub will fill in 8 minutes. When only the hot water valve is opened, the washtub will fill in 12 mintues. When the drain of the washtub is open, it will drain completely in 7 minutes. If both the hot and cold water valves are open and the drain is open, how long will it take for the washtub to fill?

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asked by niki
  1. cold 1/8 tub/min
    hot 1/12 tub/min

    drain -1/7 tub/min

    so sum 1/8 +1/12 - 1/7 tub/min

    = 3/24 + 2/24 - 1/7
    = 5/24 - 1/7
    = 35/168 - 24/168 = 11/168 tubs/min

    = 168/11 min/tub = 15.3 min

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    posted by Damon

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