A bookshelf has upper and lower layers. The number of books on the upper layer is that of the lower layer (4/5). After taking 21 books from the lower layer and putting them in the upper layer, the number of books on the upper layer is the number of books on the lower layer (7/8). There are a total of How many books.
一个书架上有上、下两层，上层书的本数是下层的(4/5)，从下层取出21本放入上层后，上层书的本数是下层的(7/8)，这个书架上共有多少本书．
<gadget id="calculator">8 + 7</gadget>
<output>15</output>

<gadget id="calculator">7 / 15</gadget>
<output>7/15 = around 0.466667</output>

<gadget id="calculator">4 + 5</gadget>
<output>9</output>

<gadget id="calculator">4 / 9</gadget>
<output>4/9 = around 0.444444</output>

<gadget id="calculator">(7/15) - (4/9)</gadget>
<output>1/45 = around 0.022222</output>

<gadget id="calculator">21 / (1/45)</gadget>
<output>945</output>

<result>945</result>