A piece of cake, mother ate (1/7), father ate (2/7), how much is left?
一块蛋糕，妈妈吃了(1/7)，爸爸吃了(2/7)，还剩下几分之几？
<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>