There is a three-story bookshelf, and the number of books on the upper floor accounts for 2/9 of the total. If 15 books are placed on the upper floor from the lower floor, the number of books stored on the three floors is equal. How many books are stored in this bookshelf?
有一个三层的书架，上层数占总数的(2/9)，如果从下层拿15本放到上层，则三层存书本数相等．这个书架存书多少本？
<gadget id="calculator">15 + 15</gadget>
<output>30</output>

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

<gadget id="calculator">2 / 9</gadget>
<output>2/9 = around 0.222222</output>

<gadget id="calculator">(4/9) - (2/9)</gadget>
<output>2/9 = around 0.222222</output>

<gadget id="calculator">30 / (2/9)</gadget>
<output>135</output>

<result>135</result>