If one goose can be exchanged for 3 chickens, and 4 chickens can be exchanged for 7 ducks, then how many ducks can be exchanged for 4 geese?
一只鹅能换3只鸡，4只鸡能换7只鸭，那么4只鹅能换多少只鸭．
<gadget id="calculator">4 / 4</gadget>
<output>1</output>

<gadget id="calculator">3 * 1</gadget>
<output>3</output>

<gadget id="calculator">7 * 3</gadget>
<output>21</output>

<result>21</result>