You can put this solution on YOUR website! I think you meant to say "sample mean" instead of "population mean". The whole goal of a confidence interval is to estimate the parameter, which in this case is the population mean. If you know the parameter, then why bother estimate it with a confidence interval?
The sample mean is xbar = 70
The confidence interval is of the form (L, U) where L is the lower endpoint and U is the upper endpoint
If the margin of error is E, then
L = xbar - E
L = 70 - 20
L = 50
and
U = xbar + E
U = 70 + 20
U = 90
Making the confidence interval to be (L, U) = (50, 90) which can be written as
Note: is the greek letter mu often used in statistics to represent the population mean
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