If the population of a variable is normal then roughly 997 o

If the population of a variable is normal, then roughly 99.7% of the values are: Within 6 standard deviations of the mean Within 4 standard deviations of the mean Within 2 standard deviations of the mean Within 3 standard deviations of the mean

Solution

(a) About 68% of the area under the normal curve is within one standard deviation of the mean.
(b) About 95% of the area under the normal curve is within two standard deviations of the mean.
(c) Practically all of the area under the normal curve is within three standard deviations of the mean.

Here 99.7~ 100% , that explains within three standard deviations of the mean.