Answer:
Mom's car can hold 18 gallons.
Dad's car can hold 12 gallons.
Explanation:
50% of 12 is 6.
If you need to know how much mom's car can hold, it is dad's amount + 6, which is 18. Mom's car can hold 18 gallons.
If it is dad's, then it is what was stated, 12 gallons.