Question 1160843
half-interval is z*sigma/sqrt(n), and this is added and subtracted to and from the mean to get the interval limits.

half-interval for a is z(0.975)*40/sqrt(64)=9.8
(40.2, 59.8)

half-interval for b is z(0.995)*20/sqrt (225).  Variance is sigma^2 so have to take the square root of 400 for sigma.
=2.576*20/15=3.43
(81.57, 88.43)

half-interval for c is z(0.95)*50/sqrt(485)
=1.645*50/sqrt (485)=3.73
(506.27, 513.73)