There is a table tennis match between every two students, how many games will the 6 students play in total?
每两名同学之间进行一场乒乓球比赛，6名同学一共要比赛多少场．
<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>