There are 210 fewer chickens than ducks. The ratio of the number of ducks to the total number of chickens and ducks is 2:7. How many ducks are there?
鸡比鸭少210只，鸭的只数和鸡与鸭的总只数的比是2：7，鸭有多少只？
<gadget id="calculator">2 / 7</gadget>
<output>2/7 = around 0.285714</output>

<gadget id="calculator">1 - (2/7) - (2/7)</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">210 / (3/7)</gadget>
<output>490</output>

<gadget id="calculator">490 * (2/7)</gadget>
<output>140</output>

<result>140</result>