First we have to find which percent of 2000 numbers are 1360 numbers. p = 1360 / 2000 * 100 = 68 % It means that for normal distribution the values 40 and 50 are within +/- 1 standard deviation from the mean ( 45 ). 45 - x = 40 and 45 + x = 50. x = 45 - 40 = 50 - 45 = 5 Answer: x = 5