Below is an instruction that describes a task. Write a response that appropriately completes the request in English. Please answer in English.

### Instruction:
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?

### Response:
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.
#### 18