A lake with constant volume V, in km3, contains a quantity of Q km3 pollutant. Clean water enters the lake and causes a total outflow of r km3 per year. The rate at which the pollutant decreases at any time t equals the product of the pollutant Q per volume V and the rate at which the water flows out of the lake. If 
km3 and 
km3 per year, how many years will it take for the pollutant to decrease to half of its original quantity? Round to the nearest whole year.
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