Chapter 1.5, Problem 63E

Marine biology. Marine life depends on the microscopic plant life that exists in the photic zone, a zone that goes to a depth where only 1% of surface light remains. In some waters with a great deal of sediment, the photic zone may go down only 15 to 20 feet. In some murky harbors, the intensity of light d feet below the surface is given approximately by

  $I=I_0{e}^{-0.23d}$

What percentage of the surface light will reach a depth of

1. (A) 10 feet?
2. (B) 20 feet?