There are direct air routes between six major cities including Nanjing, Beijing, and Shanghai. How many air routes are there in total?
在南京、北京、上海等6个大城市之间，都有直达的航空线，一共有多少条条航空线．
<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>