There are 6 classes in the third grade of Experimental Primary School. In a tug-of-war competition in the school, every two classes in the third grade have to compete once.
实验小学三年级有6个班，在一次校内拔河比赛中，三年级每两个班之间都要比赛一场，三年级6个班共要比赛多少场．
<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>