One pig can be exchanged for 2 dogs, one dog can be exchanged for 3 ducks, and 4 pigs can be exchanged for how many ducks.
一头猪可换2只狗，一只狗可换3只鸭，4头猪可换多少只鸭．
<gadget id="calculator">2 * 3</gadget>
<output>6</output>

<gadget id="calculator">6 * 4</gadget>
<output>24</output>

<result>24</result>