First find the total number of vehicles Carmen counted: 50 trucks + 40 cars = <<50+40=90>>90 vehicles
Then find the number of trucks that were red: 50 trucks / 2 = <<50/2=25>>25 trucks
Then find the number of trucks that were black: 50 trucks * .2 = <<50*.2=10>>10 trucks
Then subtract the red and black trucks from the total number of trucks to find the number of white trucks: 50 trucks - 25 trucks - 10 trucks = <<50-25-10=15>>15 trucks
Then divide the number of white trucks by the number of vehicles and multiply by 100% to find the percentage of vehicles that are white trucks: 15 trucks / 90 vehicles * 100% = 16.66...%, which rounds up to 17%.
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