bookshelf has three layers: upper, middle, and lower. The upper layer contains 72 books, which is 12 less than the middle layer. The lower layer has twice as many books as the middle layer. How many books are placed in the lower layer?
一个书架上、中、下三层，上层放书72本，比中层少12本，下层的书是中层的2倍，下层放书多少本？
<gadget id="calculator">72 + 12</gadget>
<output>84</output>

<gadget id="calculator">84 * 2</gadget>
<output>168</output>

<result>168</result>