There are two layers of books on the bookshelf, and the books on the upper layer account for (3/7) of the total. If 10 books are taken from the lower layer and put into the upper layer, then the books on the two layers are equal. How many books are there in the upper level?
书架上有两层书，上层书的本书占总数的(3/7)．若从下层取出10本放入上层，则两层本书相等．求原来上层有多少本书？
<gadget id="calculator">10 + 10</gadget>
<output>20</output>

<gadget id="calculator">3 / 7</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">1 - (3/7) - (3/7)</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">20 / (1/7)</gadget>
<output>140</output>

<gadget id="calculator">140 * (3/7)</gadget>
<output>60</output>

<result>60</result>