Hector is trying to make chocolate milk for a party. He has made a mix that is 10% chocolate. He has also made a mix that is 25% chocolate. Unfortunately, the 10% mix is too weak and the 25% mix is way too chocolaty. He believes that a 15% mix would be perfect. He has 30 gallons of the 25% mix and he wants to use it all up. How many gallons of the 10% mix should be added to the 25% mix to make a mixture that is 15% chocolate? How many gallons in total will he end up with?