There is a barrel of oil. After 30% of it is used, the weight of the barrel is 20 kg; after 50% of it is used, the weight of the barrel is 16 kg. How much does the barrel of oil weigh?
有一桶油，用去它的30%后，连桶重20千克；用去它的50%后，连桶重16千克，这桶油原来重多少千克？
<gadget id="calculator">20 - 16</gadget>
<output>4</output>

<gadget id="calculator">50 / 100</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">30 / 100</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">(1/2) - (3/10)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">4 / (1/5)</gadget>
<output>20</output>

<result>20</result>