Ralph has just returned from fishing at Big Lake. He was told that the surface of Big Lake is 31 feet above the surface of Long Lake, a neighboring lake. He has also been told that Long Lake is half as deep as Big Lake, and that the bottom of Long Lake is eight feet below the bottom of Big Lake. He is trying to figure out how deep the two lakes are. What are their depths? If B represents the depth of Big Lake, write an equation that represents the above information.