For a pile of goods, (1/4) of it is removed first, and (2/5) of it is removed later, leaving ((())/(())).
一堆货物，先运走它的(1/4)，后来又运走它的(2/5)，剩下((())/(()))．
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 - (1/4) - (2/5)</gadget>
<output>7/20 = around 0.35</output>

<result>7/20 = around 0.35</result>