After 25% of a barrel of oil is used up, there are still 30 kilograms left. How many kilograms of this barrel of oil are used?
一桶油用去25%后，还剩下30千克，这桶油用去了多少千克．
<gadget id="calculator">25 / 100</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 - (1/4)</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">30 / (3/4)</gadget>
<output>40</output>

<gadget id="calculator">40 * (1/4)</gadget>
<output>10</output>

<result>10</result>