There are 5 tons of rice, after selling 30% of the total (1/5), how many tons are left.
有5吨大米，卖出30%后，又卖出总数的(1/5)，还剩多少吨．
<gadget id="calculator">30 / 100</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (3/10) - (1/5)</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">5 * (1/2)</gadget>
<output>5/2 = around 2.5</output>

<result>5/2 = around 2.5</result>