guys, help me with this, i am still stuck, this is the last one, i am done with all of my work for today.

Consider a reservoir with a volume of 8 billion cubic feet (ft^3) and an initial pollutant concentration of 0.25%. There is a daily of 500 million ft^3 of water with a pollutant concentration of 0.05% and an equal daily outflow of the well mixed water in the reservoir. How long will it take to reduce the pollutant concentration in the reservoir to 0.10%?