There is 1 ton of coal, (1/5) is used for the first time, and (1/5) is used for the second time, how many tons of coal are left.
有1吨煤，第一次用去(1/5)，第二次用去(1/5)吨，还剩多少吨煤．
<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">1 * (4/5)</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">(4/5) - (1/5)</gadget>
<output>3/5 = around 0.6</output>

<result>3/5 = around 0.6</result>