The truck tank has 10 gallons in it because 20 x .5 = <<20*.5=10>>10
The car tank has 4 gallons in it because 12 x (1/3) = <<12*(1/3)=4>>4
She needs to add 10 gallons to the truck because 20 - 10 = <<20-10=10>>10
She needs to add 8 gallons to the car because 12 - 4 = <<12-4=8>>8
She needs to add 18 gallons in total because 10 + 8 = <<10+8=18>>18
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