10 students talk to each other on the phone, if every two students have to call each other, how many calls will be made in total.
10位同学间相互通电话，如果每两位同学之间都要通一次电话，一共要通多少次电话．
<gadget id="calculator">10 - 1</gadget>
<output>9</output>

<gadget id="calculator">9 / 2</gadget>
<output>9/2 = around 4.5</output>

<gadget id="calculator">10 * (9/2)</gadget>
<output>45</output>

<result>45</result>