Marine life is dependent upon the microscopic plant life

Marine life is dependent upon the microscopic plant life that exists in the photic zone, a zone that goes to a depth where about 1% of the surface light remains. In some waters deal of sediment, the photic zone with a great may go down only 15 to 20 feet. In some murky harbors, the intensity of light a feet below the surface is given approximately by I=I0.e^(-0.23d) What percentage of the surface light will reach a depth of: a. 10 feet? b. 20 feet?