Please help explain the formula I use for this question.

A rainstorm in Portland, Oregon, wiped out the electricity in 10% of the households in the city. Suppose that a random sample of 70 Portland households is taken after the rainstorm. Answer the following. (If necessary, consult a list of formulas.) (a) Estimate the number of households in the sample that lost electricity by giving the mean of the relevant distribution X b I) (that is, the expectation of the relevant random variable). Do not round your response. ' (b) quantify the uncertainty of your estimate by giving the standard deviation of the distribution. Round your response to at least three decimal places