This is a binomial problem
a) the mean of a binomial problem is given by np
n= number of the population
p=probability of success
in this case because the question wants the number of household that lost electricity, 5% is the number of success..
(0.05)(80)=4
4 houses lost electricity
b)
standard deviation is the square root of the variance. Variance is defined by npq
n= number of the population
p=probability of success
q=probability of failure
(80)(0.05)(0.95)=3.8 ..
square root of 3.8 is 1.949