Just do the arithmetic! Putting d= 50 and 100 in that formula gives the light intensity as a fraction times the surface light. The "percentage" is just that fraction changed to a percent.
Light intensity l relative to depth d (in feet) for one of the clearest bodies of water in the world, the Sargasso Sea in the West Indies, can be approximated by l = l0e^-0.00942d where lo is the intensity of light at the surface. What percentage of the surface light will reach a depth of A) 50 feet B) 100 feet?