Phoenix water is provided to approximately 1.43 million people, who are served through more than 362000 accounts. All accounts are metered and billed monthly. The probability that an account has an error in a month is 0.001, and accounts can be assumed to be independent.

The mean and standard deviation of the number of accounts error is:

mean = 362,000 standard deviation = 19

Explanation:

the parameters are:

n = number of accounts = 362,000 p = probability of error = 0.001

mean of the number of account errors = E (X) = np = (362,000 x 0.001) = 362

standard deviation = √[np (1 - p) ] = √[362 (1 - 0.001) ] = √[362 (0.999) ] = √361.638 = 19.02 ≈ 19