Can you explain how to solve this math problem: Timmy, Tommy and Tina are filling up a kiddie pool in their backyard.  Each has a pail they fill with water from a house before dumping it into the pool.  If Timmy's pail holds twice as much water as Tommy's, and Tommy's holds 2 gallons more than Tina's, and Tina's is 4 gallons, how much water do the three of them fill the pool with after 3 trips each?