There is a barrel of oil, 40% of which is used for the first time, and 5 kg is used less than the first time for the second time, and there is 15 kg left in the barrel.
有一桶油，第一次用去40%，第二次比第一次少用去5千克，桶中还剩15千克，这桶油原来有多少千克？
<gadget id="calculator">15 - 5</gadget>
<output>10</output>

<gadget id="calculator">40 / 100</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 - (2/5) - (2/5)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">10 / (1/5)</gadget>
<output>50</output>

<result>50</result>