6 students meet each other and say hello to each other. If every two students shake hands once, how many times do they need to shake hands in total?
6名同学见面互相问好，如果每两个同学所握一次手，一共需要握多少次手．
<gadget id="calculator">6 - 1</gadget>
<output>5</output>

<gadget id="calculator">5 / 2</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">6 * (5/2)</gadget>
<output>15</output>

<result>15</result>