in the United States, annual sales of tennis shoe

in the United States, annual sales of tennis shoes increased by $4 million from 1996 to 1997, decreased by $30 million from 1997 to 1998, decreased by $10 million from 1998 to 1999, and increased by $15 million from 1999 to 2000. In 2000, sales of tennis shoes were $520 million. Find the sales of tennis shoes in 1996.