Bob places $1,000 a year in his IRA for ten years and then invests $2,000 a year for the next ten years. Mary places $2,000 a year in her IRA for ten years and then invests $1,000 a year for the next ten years. They both have invested $30,000. If they earn 8 percent annually, how much more will Mary have earned than Bob at the end of 20 years?