Your superintendent wants to know how efficient your primary clarifier was at removing solids during a major rainstorm a few days earlier. The lab tech tells you that the average 24-hour composite TSS of the sewage entering the primary settling tank on the day in question was 228 ppm, and the average 24-hour composite TSS of the effluent that same day was 87 ppm. What do you tell the superintendent was the approximate percent removal?