There are 210 more ducks than chickens. The ratio of the number of ducks to the total number of chickens and ducks is 2:7. How many chickens are there?
鸭比鸡多210只．鸭的只数和鸡与鸭的总只数的比是2：7，鸡有多少只？
<gadget id="calculator">7 - 2</gadget>
<output>5</output>

<gadget id="calculator">5 - 2</gadget>
<output>3</output>

<gadget id="calculator">210 / 3</gadget>
<output>70</output>

<gadget id="calculator">70 * 5</gadget>
<output>350</output>

<result>350</result>