A barrel of oil, the first use (1/5), the second use (2/7), how much is left?
一桶油，第一次用去(1/5)，第二次用去(2/7)，还剩多少？
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">2 / 7</gadget>
<output>2/7 = around 0.285714</output>

<gadget id="calculator">1 - (1/5) - (2/7)</gadget>
<output>18/35 = around 0.514286</output>

<result>18/35 = around 0.514286</result>