5 children make one phone call between each of them, how many calls will be made in total.
5个小朋友每两个人之间通一次电话，一共要通多少次电话．
<gadget id="calculator">5 - 1</gadget>
<output>4</output>

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

<gadget id="calculator">4 * (5/2)</gadget>
<output>10</output>

<result>10</result>