Question 970146
68% of the distribution lies within one standard deviation of the mean. about 95% is within 2 stdv. 

So add and subtract 12 to each side of the mean. This will get you the interval of 298 to 322.

286 + 12 = 298 so another standard deviation on the lower side of the interval is needed. 

Subtract the 1 stdv from 2 stdv to find how much more of the data 2 stdvs includes. 
95% - 68% = 27%
Divide 27% by 2 because we only need the left. 
27% ÷2 = 13.5% 

Finally add 13.5% to 1 stdv to get the total. 
13.5% + 68% = 81.5% for the interval from 286 to 322. 
Rounded to 82%.