There are 6 classes in the fifth and sixth grades of Qingcheng Primary School, and there is a basketball game between every two classes. How many games will be played in total.
青城小学五年级、六年级一共有6个班级，每两个班级之间进行一场篮球比赛．一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>