A pile of sand, use (2/7) for the first time, use (1/4) for the second time, how much of this pile of sand is left.
一堆沙子，第一次用去(2/7)，第二次用去(1/4)，还剩下这堆沙子的多少．
<gadget id="calculator">2 / 7</gadget>
<output>2/7 = around 0.285714</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 - (2/7) - (1/4)</gadget>
<output>13/28 = around 0.464286</output>

<result>13/28 = around 0.464286</result>