A barrel of oil weighs 50 kilograms, and if (1/5) is used up, how much more kilograms are left over than used.
一桶油重50千克，用去了(1/5)，剩下的比用去的多多少千克．
<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">50 * (4/5)</gadget>
<output>40</output>

<gadget id="calculator">50 * (1/5)</gadget>
<output>10</output>

<gadget id="calculator">40 - 10</gadget>
<output>30</output>

<result>30</result>