How long will it take before the beach must be closed?

A local beach is monitored each year for chemical pollutants. Currently, the concentration of a certain hazardous chemical is 6 ppm (parts per million). The maximum safe level of this chemical is 20 ppm. If the concentration increases at 5 percent per year, how long will it take before the beach must be closed? I am suppose to use the FV function in Excel 2007.