A barrel of oil weighs 5kg, and oil accounts for 90% of the total mass. After using up some oil, the oil accounts for 80% of the total mass. How many kilograms does this barrel of oil weigh?
一桶油重5kg，油占总质量的90%，用掉一些后，油占总质量的80%，这时这桶油重多少千克？
<gadget id="calculator">90 / 100</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">1 - (9/10)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">1 - (4/5)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/10) / (1/5)</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">5 * (1/2)</gadget>
<output>5/2 = around 2.5</output>

<result>5/2 = around 2.5</result>