The amount of time from 8:30 a.m. to 11:15 a.m is 2 hours and 45 minutes, or 2 + (45/60). To reduce the fraction, divide 45 by 60 to get 0.75, or \(\frac{{3} }{{4}}\). The total amount of time Margaret has to get ready is \(2\frac{{3} }{{4}}\) hours.