Uncle Zhang raises a total of 840 chickens and ducks, and the number of chickens is 3/4 that of ducks. How many ducks does Uncle Zhang raise?
张大爷养鸡和鸭共有840只，鸡的只数是鸭的(3/4)．张大爷养了多少只鸭？
<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">1 + (3/4)</gadget>
<output>7/4 = around 1.75</output>

<gadget id="calculator">840 / (7/4)</gadget>
<output>480</output>

<result>480</result>