(1/7) of a farmland is planted with tomatoes, (2/7) is planted with corn, and the rest is planted with sweet potatoes. What fraction of the whole farmland is planted with sweet potatoes?
一块农田的(1/7)种了西红柿，(2/7)种了玉米，剩下的种红薯，种红薯的地占整块农田的几分之几？
<gadget id="calculator">1 / 7</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">2 / 7</gadget>
<output>2/7 = around 0.285714</output>

<gadget id="calculator">1 - (1/7) - (2/7)</gadget>
<output>4/7 = around 0.571429</output>

<result>4/7 = around 0.571429</result>