If 20 rabbits can be exchanged for 2 sheep, and 9 sheep can be exchanged for 3 pigs, how many rabbits can one pig exchange for?
假如20只兔子可换2只羊，9只羊可换3头猪，那么一头猪可换多少只兔子．
<gadget id="calculator">3 * 10</gadget>
<output>30</output>

<result>30</result>