Question 1166765
this is a binomial with n=50 and p=0.20 (failure)
would expect 10 (50*).20 to have taken more than 30 minutes.
exactly 8 taking more is 50C8*0.20^8*0.80^42 or probability 0.1169
fewer than 8 would be <= 7 and that probability would be 0.1904

calc binompdf(50,0.2.8) for the first
binomcdf (50,0.2,7) for the second.

normal approximation mean is 10
variance is 10*0.8 or 8
sd is sqrt(8) or 2.828
less than 8 would be z<(7.5-10)/2.828  or -0.88 and that probability is 0.1894, a decent estimate.