A bag of chemical fertilizer, use up (2/7) for the first time, use up (1/5) for the second time, what fraction of the bag of fertilizer is left?
一袋化肥，第一次用去它的(2/7)，第二次用去它的(1/5)，还剩下这袋化肥的几分之几？
<gadget id="calculator">2 / 7</gadget>
<output>2/7 = around 0.285714</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

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

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