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 = I₀e ^{- 0.23d}

where I₀ 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?