Chapter 3 - The Normal Distributions - Check Your Skills - Page 95: 3.20

C) 2.5%

$(130-100)/15 = z$ $30/15 = z$ $z = 2$ Under the 68-95-99.7 rule, 95% of all observations would be within two standard deviations of the mean. However, we are looking for half of that percentage (we don't want to include the population 2 or more standard deviations from the mean that are below the mean). $5/2 = 2.5%$