In environmental engineering (a specialty area in civil engineering), the following equation can be used to compute the oxygen level c (mg/L) in a river downstream from a sewage discharge:

c = 10 – 20 (e^–0.15x – e^–0.5x)

Where x is the distance downstream in kilometers

(a) Determine the distance downstream where the oxygen level first falls to a reading of 5 mg/L. Determine your answer to a 1% error. Note that levels of oxygen below 5 mg/L are generally harmful to gamefish such as trout and salmon.

(b) Determine the distance downstream at which the oxygen is at a minimum. What is the concentration at that location?