There is a piece of cardboard. Mr. Wang used this paper (2/7) to make number cards and (4/7) to make test cards. What fraction of this piece of cardboard is left?
有一张硬纸板，王老师做数字卡片用了这张纸的(2/7)，做试题卡片用了这张纸的(4/7)．还剩下这张纸板的几分之几？
<gadget id="calculator">2 / 7</gadget>
<output>2/7 = around 0.285714</output>

<gadget id="calculator">4 / 7</gadget>
<output>4/7 = around 0.571429</output>

<gadget id="calculator">1 - (2/7) - (4/7)</gadget>
<output>1/7 = around 0.142857</output>

<result>1/7 = around 0.142857</result>