There are 7 dishes in the cafeteria. Each student can only and must choose 2 kinds of dishes at a time. How many different choices are there?
食堂共有7个菜，每位同学一次只能且必须选2种菜，则一共有多少种不同选法．
<gadget id="calculator">7 - 1</gadget>
<output>6</output>

<gadget id="calculator">7 / 2</gadget>
<output>7/2 = around 3.5</output>

<gadget id="calculator">6 * (7/2)</gadget>
<output>21</output>

<result>21</result>