There is a barrel of oil, 2.1 kilograms are taken out for the first time, and the rest (1/4) is taken out for the second time. At this time, there are still 7.5 kilograms in the barrel. How many kilograms is there in the barrel?
有一桶油，第一次取出2.1千克，第2次取出余下的(1/4)，这时桶里还有7.5千克，这桶油原来有多少千克？
<gadget id="calculator">1 / 4</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">7.5 / (3/4)</gadget>
<output>10</output>

<gadget id="calculator">10 + 2.1</gadget>
<output>12.1</output>

<result>12.1</result>