There are 210 chickens. The ratio of the number of chickens to the total number of chickens and ducks is 2:7. How many ducks are there?
鸡有210只，鸡的只数和鸡与鸭的总只数的比是2：7，鸭有多少只？
<gadget id="calculator">5 / 7</gadget>
<output>5/7 = around 0.714286</output>

<gadget id="calculator">1 - (5/7)</gadget>
<output>2/7 = around 0.285714</output>

<gadget id="calculator">210 / (2/7)</gadget>
<output>735</output>

<gadget id="calculator">735 * (5/7)</gadget>
<output>525</output>

<result>525</result>