There are 7 students talking on the phone, every two people made a phone call, how many times did they make a phone call in total?
有7名同学通电话，每两人通了一次电话，一共通了多少次电话．
<gadget id="calculator">6 / 2</gadget>
<output>3</output>

<gadget id="calculator">7 * 3</gadget>
<output>21</output>

<result>21</result>