How to get the answer 0.89?

8. The number of adults living in homes on a randomly selected city block is described by the following probability distribution. Number of adults, x 2 3 4 Probability, P(x) 0:25 0.50 0.15 0.10 What is the population standard deviation of the probability distribution? (a) 0.5 (b) 0.62 (c) 0.79 1 - (d) 0.89