This is a chemistry question:

Grandma wanted to make a big batch of lemonade for a family reunion. She knew she had two gallons of lemonade that were 20% sugar. She wanted the final product to be a little less sweet, so she mixed up several gallons of lemonade that were only 10% sugar. How much of the 10%-sugar lemonade should Grandma mix with the two gallons of 20%-sugar lemonade to end up with lemonade that is 14% sugar?