For a barrel of oil, (3/7) is used the first time, and (1/7) is used more than the second time, what fraction of the barrel of oil is used twice?
一桶油，第一次用去它的(3/7)，比第二次多用去(1/7)，两次用去这桶油的几分之几？
<gadget id="calculator">3 / 7</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">1 / 7</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">(3/7) - (1/7)</gadget>
<output>2/7 = around 0.285714</output>

<gadget id="calculator">(2/7) + (3/7)</gadget>
<output>5/7 = around 0.714286</output>

<result>5/7 = around 0.714286</result>