Question 1121170
Need a z-score with all of these, which essentially makes the table needed a standard normal table with mean 0 and sd 1
z=(x-mean)/sd
a.  (275-280)/20=-0.25
want the probability of z <-0.25, and that is 0.4013 or 40.13%

b.(243-280)/20=-1.85, and probability of z < -1.85 is 0.0322 or 3.22%

c.  this is z of -39/20 or -1.95 and z of 21.60/20 or 1.08.  This has a probability of 0.8343 or 83.43%
Use 2nd VARS 2 for normal cdf and put in the numbers, with at least -6 or +6 for minus or plus infinity. 1E99 is fine but isn't really necessary.