A barrel of oil weighs 20 kg, 50% of it is used for the first time, and 0.5 kg is used for the second time, how many kg is lighter than the original.
一桶油重20千克，第一次用去它的50%，第二次用去0.5千克，比原来轻了多少千克．
<gadget id="calculator">50 / 100</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">20 * (1/2)</gadget>
<output>10</output>

<gadget id="calculator">10 + 0.5</gadget>
<output>10.5</output>

<result>10.5</result>