1. The problem statement, all variables and given/known data When a light ray goes trough a transparent surface, its intensity rate of change decreases proportionaly to I(t), where t is the thickness. On a specific kind of water, intensity 3 feet from below the surface is 25% its original intensity. What is the intensity from 15 ft below the surface? (sorry for my english lol) 2. Relevant equations dI/dt = kI, I(3) = 0.25I_0 3. The attempt at a solution My attempt is here: http://img69.imageshack.us/img69/4237/zill2.png [Broken] The correct answer is I = 0.0009765625 I_0, but I don't know how to get there!!! Any hint please?