in the normal distribution with any given mean and standard deviation, we know that approximately 68% of the observations fall within one standard deviation of the mean 95% of the observations fall within two standard deviations of the mean 99.7% of the observations fall within 3 standard deviations of the mean. this is sometimes called the 68-95-99.7 empirical rule of thumb. use this rule to answer the following: a given exam has a normal distribution with a mean of 70 and a standard deviation of 10. a sample of size 25 is selected. what percentage of the time would you expect the mean of this sample size to fall between 66 and 72