A barrel of oil, use up its (1/6) for the first time, and use up (1/4) for the second time. If 3 kilograms are used up for the third time, how many kilograms are used for the first time?
一桶油，第一次用去它的(1/6)，第二次用去它的(1/4)．如果第三次用去3千克正好用完，那么第一次用去多少千克？
<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 - (1/6) - (1/4)</gadget>
<output>7/12 = around 0.583333</output>

<gadget id="calculator">3 / (7/12)</gadget>
<output>36/7 = around 5.142857</output>

<gadget id="calculator">(36/7) * (1/6)</gadget>
<output>6/7 = around 0.857143</output>

<result>6/7 = around 0.857143</result>