10 basketball teams play a single round robin game, with every two teams playing one game. How many games are needed.
10个篮球队进行单循环比赛，每两个队要赛一场．共需比赛多少场．
<gadget id="calculator">10 - 1</gadget>
<output>9</output>

<gadget id="calculator">9 / 2</gadget>
<output>9/2 = around 4.5</output>

<gadget id="calculator">10 * (9/2)</gadget>
<output>45</output>

<result>45</result>