The school held a table tennis selection competition. Each contestant had to compete with all other players. There were 21 matches in total. How many people participated in the selection competition? How many
学校进行乒乓球选拔赛，每个参赛选手都要和其他所有选手各赛一场，一共进行了21场比赛，有多少人参加了选拔赛？多少
<result>7</result>