Question 1121416
z=(x-mean)/sd with mean 80 min and sd 10 min
z<-20/10 or -2 for 1 hour or less.  That probability is 0.0228

60 to 75 minutes is z between 0 and +1.5 or 0.4332

90 minutes is 3 sd and the probability z>3 is 0.001
over 60 students, this would be no students