Problem N5618 probability theory confidence interval

5618. 400 pieces were randomly selected from a large batch of electric lamps. to determine the average burning time. The sample average burning time of the lamps turned out to be 1220 hours. Let us find, with a confidence factor γ = 0.997, a confidence interval for the average burning time of an electric lamp over the entire batch, if the standard deviation of the burning time is 35 hours.