Can you explain how to solve this math problem: Christian and Sue wanted to get a $50.00 bottle of perfume for their mom for her birthday.  Christian had $5.00 saved up and Sue had $7.00.  Christian then mowed 4 of his neighbors' yards, charging $5.00 each, while Sue walked 6 dogs for her neighbors, charging $2.00 per dog.  How much more money will Christian and Sue need to make in order to buy the bottle of perfume for their mom?