Rachel is painting the walls of her bedroom. She makes a pink color of paint by mixing some cups of red paint into some white paint. She added three
cups of red paint to four gallons of white paint to get the perfect pink color,

Question: Rachel wants to make more of the pink paint that matches the color
she made before. She starts with 10 gallons of white paint. How many cups of
red paint should Rachel add so that the pink color matches?