The intensity L(x) of ligh X feet beneath the surface of the ocean satisfies the differential equation: dL/dx = -kL As a diver, you know from experience that diving to 18 ft in the Caribbean Sea cuts the intensity in half. You cannot work without artificial light when the intensity falls below one-tenth of the surface value. About how deep can you expect to work without artificial light? We are learning compound interest and whatnot. So, the equation we manipulate is: A = Ao(e^(rt)) A is probably a tenth of (-kL), which is the original surface value of the ocea. I somehow should use the ratio of 18ft cutting the intensity in half, but I'm not sure how.