There are 500 tons of coal in one batch, and (1/5) of the total amount was used up in the first month. How many tons of coal are left useless?
一批煤共有500吨，第一个月用去了总量的(1/5)，还剩下多少吨煤没用？
<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">500 * (4/5)</gadget>
<output>400</output>

<result>400</result>