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Counting by Units of Standard Deviation

Though we don't need to know how to calculate standard deviation, we do need to know how to use it. The standard deviation of a certain list is always expressed along with the average of the same list.

For example, one might say that the average gambler's winning in Las Vegas is $200, with a standard deviation of $30. It is then possible to count in units of standard deviation. One standard deviation would be $30, two standard deviations would be 2 x 30 = $60, and three standard deviations would be 3 x 30 = $90.

Example

Farmers in the United States grew an average (arithmetic mean) of 80 tons of corn each, with a standard deviation of 10 tons. What value is two standard deviations away from the mean?