The electric cooperative needs to know the mean household us

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 0.09 kWh. A previous study found that for an average family the variance is 5.76 kWh and the mean is 16.6 kWh per day.

Solution

Note that      
      
n = z(alpha/2)^2 s^2 / E^2      
      
where      
      
alpha/2 = (1 - confidence level)/2 =    0.01  
      
Using a table/technology,      
      
z(alpha/2) =    2.326347874  
      
Also,      
      
s = sample standard deviation =    2.4  
E = margin of error =    0.09  
      
Thus,      
      
n =    3848.458262  
      
Rounding up,      
      
n =    3849   [ANSWER]