There are 64 sets of books on the upper and lower layers of the bookshelf. The number of sets on the lower layer is 20 times less than that on the upper layer. How many sets of books are there on the lower layer?
书架上有上、下两层书共64套，下层的套数比上层的2倍少20套同样的书，下层放了多少套书？
<gadget id="calculator">64 + 20</gadget>
<output>84</output>

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

<gadget id="calculator">84 / 3</gadget>
<output>28</output>

<gadget id="calculator">64 - 28</gadget>
<output>36</output>

<result>36</result>