The city of Madison regularly checks the quality of water at swimming beaches located on area lakes. Fifteen times the concentration of fecal coliforms, in number of colony forming units (CFU) per 100 ml of water, was measured during the summer at one beach.

180 1600 90 140 50 260 400 90 380 110 10 60 20 340 80

(a) Calculate the sample variance.

(b) Calculate the sample standard deviation.

(c) One day, the water quality was bad—the reading was 1600 CFU—and the beach was closed. Drop this value and calculate the sample standard deviation for the days where the water quality was suitable for swimming. Comment on the change.