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

Price Rate of Return Probability

$14 30% 0.30

22 10% 0.10

24 +20% 0..10

28 +40% 0.50

Assuming that XYZ is not expected to pay any dividends during the coming year, determine the standard deviation of possible rates of return on XYZ stock (to the nearest tenth of a percent).