First find the total distance she drives to get to the dermatologist and back to her house: 30 miles * 2 = <<30*2=60>>60 miles
Then find the total distance she drives to get to the gynecologist and back to her house: 50 miles * 2 = <<50*2=100>>100 miles
Then add those two amounts to find how far she drives total: 60 miles + 100 miles = <<60+100=160>>160 miles
Then divide that distance by her car's gas mileage to find how many gallons of gas she needs: 160 miles / 20 miles/gallon = <<160/20=8>>8 gallons
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