Let X=distribution of times. Let T=time she takes to fulfill this requirement.
We want P(X>T) =0.05. So we want P (Z>(T-20)/3=0.05. Then use tables so you will get a value for (T-20)/3
The time it takes Jessica to bicycle to school is normally distributed with a average of 20 minutes and a variance of 9 minutes. Jessica has to be at school at 8:15AM. What time should she leave the house so she will be late only 5% of the time?