Give the step-by-step reasoning process and then the final answer. Wendy's truck has a gas tank that can hold 20 gallons. She also has a car with a gas tank that holds 12 gallons. The truck's tank is half full. The car's tank is 1/3 full. If she fills them both up completely, how many gallons does she add?
The truck tank has 10 gallons in it because 20 x .5 = 10. The car tank has 4 gallons in it because 12 x (1 / 3) = 4. She needs to add 10 gallons to the truck because 20 - 10 = 10. She needs to add 8 gallons to the car because 12 - 4 = 8. She needs to add 18 gallons in total because 10 + 8 = 18.
The final answer: 18.