6 basketball teams play a single round robin game, with every two teams fortressing one game. How many games are needed.
6个篮球队进行单循环比赛，每两个队要塞一场．共需比赛多少场．
<gadget id="calculator">6 - 1</gadget>
<output>5</output>

<gadget id="calculator">5 / 2</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">6 * (5/2)</gadget>
<output>15</output>

<result>15</result>