There was a cake, father and mother each ate (2/7), and Zhang Xu ate the rest. How much of this cake did Zhang Xu eat?
有一个蛋糕，爸爸和妈妈各吃了这个蛋糕的(2/7)，剩下的被张旭吃完了．张旭吃了这个蛋糕的几分之几？
<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>

<result>3/7 = around 0.428571</result>