The books on the second floor of the bookshelf belong to the first floor (2/7). If 15 books are taken from the first floor and placed on the second floor, the ratio of the number of books on the first floor to the second floor is 2:1. How many books were placed on the first floor?
书架上第二层的书是第一层的(2/7)，如果从第一层中取出15册放入第二层，那么第一层与第二曾的册数比是2：1，问原来第一层放多少册书？
<gadget id="calculator">7 + 2</gadget>
<output>9</output>

<gadget id="calculator">7 / 9</gadget>
<output>7/9 = around 0.777778</output>

<gadget id="calculator">2 + 1</gadget>
<output>3</output>

<gadget id="calculator">2 / 3</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">(7/9) - (2/3)</gadget>
<output>1/9 = around 0.111111</output>

<gadget id="calculator">15 / (1/9)</gadget>
<output>135</output>

<gadget id="calculator">135 * (7/9)</gadget>
<output>105</output>

<result>105</result>