A pile of sand is used first (1/5), and then the rest (1/5), how much of the pile of sand is left.
一堆沙子先用去它的(1/5)，再用去剩下的(1/5)，还剩这堆沙子的多少．
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (1/5)</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">(4/5) * (4/5)</gadget>
<output>16/25 = around 0.64</output>

<result>16/25 = around 0.64</result>