Wouldn’t you be able to just skip steps 3 & 4 and just take (15mg/L-10.64mg/L)/15mg/L to get 29%? There’s a lot of extra factors that get divided out in converting to mass.
So the plant discharges 30 cfs into a stream that typically carries 2.5 cfs? That seems a little strange from a design point of view. Usually these problems have the discharge flow rate several orders of magnitude lower than the stream flow rate.
Yes, correct. This video hurts to watch. The problem is super unrealistic, and is also solved poorly. @brittanydarnell6546's comment describes how you should be solving this problem.