30% of a barrel of oil is used up, and 2.1kg is used up. How many kg does the barrel of oil have?
一桶油用去30%，用去了2.1kg，这桶油原有多少kg．
<gadget id="calculator">30 / 100</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">2.1 / (3/10)</gadget>
<output>7</output>

<result>7</result>