A12-gallon tub has a faucet that lets water in at a rate of 3 gallons per minute, and a drain that lets water out at a rate of 1.5 gallons per minute. if you start with 3 gallons of water in the tub, how long will it take to fill the tub completely?

Vol of water required to fill up the tub = 12-3 = 9
Rate of water per min  =    3 -1.5  = 1.5/min
Using the formula           vol   =   rt
                                            t = v/r
                                            t =    9/1.5    =   6
  time taken  =    6 mins