In a two-layer bookshelf, the number of books on the lower layer is 1.5 times that of the upper layer. If 15 books are taken from the lower layer and placed on the upper layer, the number of books on the two layers is equal. How many books were originally placed on the lower layer?
一个两层的书架，下层放的书是上层的1.5倍，如果从下层取15本放入上层，则两层书的本数相等，下层原来放书多少本？
<gadget id="calculator">15 + 15</gadget>
<output>30</output>

<gadget id="calculator">1.5 - 1</gadget>
<output>0.5</output>

<gadget id="calculator">30 / 0.5</gadget>
<output>60</output>

<gadget id="calculator">60 * 1.5</gadget>
<output>90</output>

<result>90</result>