There are 10 tons 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10吨煤，第一次用去(1/5)，第二次用去(1/5)吨，还剩下多少吨煤．
<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">10 * (4/5)</gadget>
<output>8</output>

<gadget id="calculator">8 - (1/5)</gadget>
<output>39/5 = around 7.8</output>

<result>39/5 = around 7.8</result>