If 6 basketball teams are to be played in groups, and every two teams must play one game, then how many games will be played in total?
把6个篮球队进行小组比赛，每两队必须进行一场比赛，那么共要进行多少场比赛．
<gadget id="calculator">6 - 1</gadget>
<output>5</output>

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

<gadget id="calculator">5 * 3</gadget>
<output>15</output>

<result>15</result>