Question

Unysis.com Is one of the most frewuented business to business web sites. Assuming that the duration of a visit to this website is normally distributed with a mean of 65.7 minutes and a standard deviation of 15 minutes

Question : Less than how many minutes will only 20% of the visits last?

Answer 1

less than 20% correspond to z-score of -0.84. So the answer is
65.7+15*(-0.84)=53.08. So it'll last less than 53.08 min.