please help me regarding this one

QUESTION 4 An astronomer wants to measure the distance from his observatory to a distant star. However, due to atmospheric disturbances, any measurement will not yield the exact distance. As a result, the astronomer has decided to make a series of measurements and then use their average value as an estimate of the actual distance. Ifthe astronomer believes that the values of the successive measurements are independent random variables that follow approximately a normal distribution with a mean of 19 light years and a standard deviation of 2 light years, how many series of measurements he needs to make so that at least 95 percent of the mean distance is less than 9.5 light years