For a full barrel of oil, 30% of it is used first, and then 30 kg is poured in. At this time, the oil in the barrel is 10% less than the original one. How many kilograms of oil need to be poured in to fill the barrel?
一满桶油，先用去30%，又倒进30千克，这时桶里的油比原来少10%，要装满这个桶，还需要倒入多少千克油？
<gadget id="calculator">30 / 100</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">10 / 100</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">(3/10) - (1/10)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">30 / (1/5)</gadget>
<output>150</output>

<gadget id="calculator">150 * (1/10)</gadget>
<output>15</output>

<result>15</result>