7 good friends haven't seen each other for a long time. When they meet, if every two people shake hands once, how many times did they shake hands in total?
7个好朋友很久没有见面了，见面时，如果每两个人握一次手，他们一共握了多少次手．
<gadget id="calculator">7 - 1</gadget>
<output>6</output>

<gadget id="calculator">7 / 2</gadget>
<output>7/2 = around 3.5</output>

<gadget id="calculator">6 * (7/2)</gadget>
<output>21</output>

<result>21</result>