The Central Limit Theorem estabilishes that, for a random variable X, with mean and standard deviation , a large sample size can be approximated to a normal distribution with mean and standard deviation .
In this problem
We are working with a sample of 81 adults, so the point estimae of the mean is the mean number of weekly hours that U.S. adults use computers at home.
20,000 x 25 = 500,000 500,000 divided by 100 = 5,000 members are lost. Members left = 15,000 Members added = 10,000 Members every year = Members left + Members added = 15,000 + 10,000 = 25,000