1 cow can be exchanged for 6 pigs, 2 pigs can be exchanged for 10 sheep, 3 sheep can be exchanged for 20 chickens, and 400 chickens can be exchanged for how many cows.
1头牛可换6头猪，2头猪可换10只羊，3只羊可换20只鸡，400只鸡可换多少头牛．
<gadget id="calculator">400 / 20</gadget>
<output>20</output>

<gadget id="calculator">20 * 3</gadget>
<output>60</output>

<gadget id="calculator">60 / 10</gadget>
<output>6</output>

<gadget id="calculator">6 * 2</gadget>
<output>12</output>

<gadget id="calculator">12 / 6</gadget>
<output>2</output>

<result>2</result>