Hi,

tank that measures 30 feet wide x 30 feet long x 21 feet deep. V = 

  900*21ft = 18,900  and 900*(.80*21) = 900* 16.8ft =  15,120

     21 - 16.8 = 4.2ft = 48in + 2.4in  = 50.4 inches below the top of the tank 

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