Chapter 2.5, 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

$I=I_0{e}^{-0.23d}$

where $I_0$ is the intensity of light at the surface. What percentage of the surface light will reach a depth of

(A) $10$ feet? (B) $20$ feet?