A piece of cake, the elder brother ate (3/7) and the younger brother ate (2/7), how much of the cake did they eat together? Leave the rest to my mother, how much of this cake did my mother eat?
一块蛋糕，哥哥吃了(3/7)，弟弟吃了(2/7)，他俩一共吃了这块蛋糕的几分之几？其余的全部留给妈妈吃，妈妈吃了这块蛋糕的几分之几？
<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>