Phoenix company common stock is currently selling for $20 per share. Security analysts have assigned the following probability distribution to the price of (and rate of return on) Phoenix stock one year from now.

Price is $16, 20, 24, and 28. Rate of return is -20%, 0%, +20%, +40%. And probability of 0.25, 0.30, 0.25, 0.20 Determine the standard deviation of possible rates of return on Phoenix stock ( to the nearest tenth of percent).