For one moon cake, my elder brother ate the whole (2/7), and my younger sister ate the whole (3/7), what fraction of this moon cake is left?
一个月饼，哥哥吃了整个的(2/7)，妹妹吃了整个的(3/7)，这个月饼还剩几分之几？
<gadget id="calculator">2 / 7</gadget>
<output>2/7 = around 0.285714</output>

<gadget id="calculator">3 / 7</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">(2/7) + (3/7)</gadget>
<output>5/7 = around 0.714286</output>

<gadget id="calculator">1 - (5/7)</gadget>
<output>2/7 = around 0.285714</output>

<result>2/7 = around 0.285714</result>