How many ways are there to choose two of 6 children to play in a group?
在6个小朋友中选择两个作一组做游戏，一共有多少种选法．
<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>