7 students are playing table tennis. Every two students have to play a game. How many games are there in total?
7名同学进行乒乓球比赛，每两名同学之间都要进行一场比赛，一共要多少场．
<gadget id="calculator">7 - 1</gadget>
<output>6</output>

<gadget id="calculator">6 / 2</gadget>
<output>3</output>

<gadget id="calculator">7 * 3</gadget>
<output>21</output>

<result>21</result>