There are several black pieces and white pieces in a box. If one black piece is taken out, the ratio of the number of remaining black pieces to the number of white pieces is 9:7. The ratio of the number of white pieces is 7:5, so how many more black pieces are there in the box than white pieces?
一个盒子里有黑棋子和白棋子若干粒，若取出一粒黑子，则余下的黑子数与白子数之比为9：7，若放回黑子，再取出一粒白子，则余下的黑子数与白子数之比为7：5，那么盒子里原有的黑子数比白子数多多少个．
<gadget id="calculator">9 / 7</gadget>
<output>9/7 = around 1.285714</output>

<gadget id="calculator">7 / 5</gadget>
<output>7/5 = around 1.4</output>

<gadget id="calculator">1 + (7/5)</gadget>
<output>12/5 = around 2.4</output>

<gadget id="calculator">(7/5) - (9/7)</gadget>
<output>4/35 = around 0.114286</output>

<gadget id="calculator">(12/5) / (4/35)</gadget>
<output>21</output>

<gadget id="calculator">(9/7) * 21</gadget>
<output>27</output>

<gadget id="calculator">1 - 21</gadget>
<output>-20</output>

<gadget id="calculator">27 + (-20)</gadget>
<output>7</output>

<result>7</result>