There are 10 class reunions, if everyone shakes hands with everyone else when they meet, how many handshakes are there in total?
有10个同学聚会，见面时如果每人都和其余的每个人握一次手，那么共握手多少次．
<gadget id="calculator">10 - 1</gadget>
<output>9</output>

<gadget id="calculator">10 / 2</gadget>
<output>5</output>

<gadget id="calculator">9 * 5</gadget>
<output>45</output>

<result>45</result>