The empirical rule states that 99.7% of the data observed following a normal distribution lies within 3 standard deviations of the mean.
Given data:
Mean: 39oz
Standard deviation: 3oz
Multiply the standard deviation by 3:
Add and subtract the result above to the mean to get the interval of 99.7% of the data;
Then, the 97.7% of the widget weights lie between 30 ounces and 48 ounces