Purification of water for drinking using UV light is a viable way to provide potable water in many areas of the world. Experimentally, the decrease in UV light of wavelength 250 nm follows the empirical relation I /I₀ = e^{−ε ′l}, where l is the distance that the light passed through the water and ε ′ is an effective absorption coefficient. ε ′ = 0.070 cm⁻¹ for pure water and 0.30 cm⁻¹ for water exiting a waste water treatment plant. What distance corresponds to a decrease in I of 15% from its incident value for 

a. Pure water

b. Waste water?