There are 210 ducks, and the ratio of the number of ducks to the total number of chickens and ducks is 5:7. How many chickens are there?
鸭有210只，鸭的只数和鸡与鸭的总只数的比是5：7．鸡有多少只？
<gadget id="calculator">210 / 5</gadget>
<output>42</output>

<gadget id="calculator">7 - 5</gadget>
<output>2</output>

<gadget id="calculator">42 * 2</gadget>
<output>84</output>

<result>84</result>