the electric cooperative needs to know the mean household usage of electricity by its non-commercial customers in kwh per day. they would like the estimate to have a maximum error of 0.12 kwh. a previous study found that for an average family the variance is 2.56 kwh and the mean is 15.5 kwh per day. if they are using a 98% level of confidence, how large of a sample is required to estimate the mean usage of electricity? round your answer up to the next integer.