For a barrel of oil, (1/4) is used for the first time, which is exactly 5 liters, and (1/2) of this barrel of oil is used for the second time, how many liters are used for the second time.
一桶油，第一次用去(1/4)，正好是5升，第二次用去这桶油的(1/2)，第二次用去多少升．
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

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

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

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

<result>10</result>