Statistics
posted by Pat .
How large a sample of U.S. adults is needed in order to estimate U with a 95% confidence interval of length 1.2 hours?

I assume this was part of your previous post:
A study was conducted in order to estimate u, the mean number of weekly hours that U.S. adults use computers at home. Suppose a random sample of 81 U.S. adults give a mean weekly computer usage time of 8.5 hours and that from prior studies , the population standard deviation is assumed to be 3.6 hours.
If so, use this formula:
Margin of error = zvalue * sd/√n
With your data:
1.2 = 1.96 * 3.6/√n
Solve for n. 
136