Answer::
If the author assumes normal range is within one S.D. (5 years) then
the low normal is Average minus 5 and high normal is Average plus 5
Low = 20.2 - 5 = 15.2
High = 20.2 + 5 = 25.2
A normal chart shows that 68.27% of values lie with one standard deviation on either side of the mean
