2 sheep can be exchanged for 1 pig, 5 pigs can be exchanged for 1 horse, and 2 horses can be exchanged for how many sheep.
2只羊可换一头猪，5头猪可换1匹马，2匹马可换多少只羊．
<gadget id="calculator">5 * 2</gadget>
<output>10</output>

<gadget id="calculator">2 * 10</gadget>
<output>20</output>

<result>20</result>