Question 1043029
The z-valueis (x-mean)/sd
Less than 20 days is (20-22)/2=-1, and it is the probability z is less than -1 or 0.1587.
Between 18 and 22 days is z between -2 and  0, and that is 0.4772.
More than 24 days is z >1, and that is the same as z<-1, or 0.1587.