A pile of coal weighs 4 tons, (1/5) is used up, and (1/5) tons are used up, how many tons are left.
一堆煤重4吨，用去(1/5)，再用去(1/5)吨，还剩多少吨．
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

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

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

<result>3</result>