Question 1167441
for z=(+/- 1.38) there is 83.27% of the area. 
Look at the table for half the distance between 83.27% and 100% and that is 91.63%.  Where the probability is 0.9162 is 1.38

half-interval is 1.38*40/sqrt(n)=9, sigma/sqrt(n)*z=error of measurement
1.38*40/9=sqrt(n)=6.13
n=37.62 or 38, rounding up.