There are 6 classes in the sixth grade of a certain elementary school. Each class sends a basketball team to compete. Every two teams compete once. How many games will there be in total?
某小学六年级有6个班，每班派出一个篮球队进行比赛，每两队比赛一场，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>