First find the time to drive to the airport: 10 minutes * 5 = <<10*5=50>>50 minutes
Then find the time it takes to get through security: 15 minutes * 3 = <<15*3=45>>45 minutes
Then find the time Mary spends waiting for the plane to take off: 20 minutes * 2 = <<20*2=40>>40 minutes
Then add the time to each step to find the total time: 10 minutes + 50 minutes + 15 minutes + 45 minutes + 20 minutes + 40 minutes = <<10+50+15+45+20+40=180>>180 minutes
Then divide that amount by the number of minutes per hour to cover the total time to hours: 180 minutes / 60 minutes/hour = <<180/60=3>>3 hours
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