There are 80 books in the lower shelf, which is 12 more books than the upper shelf. How many books are there in the upper shelf?
书架下层有80本书，比上层的4倍还多12本，这个书架上层有书多少本？
<gadget id="calculator">80 - 12</gadget>
<output>68</output>

<gadget id="calculator">68 / 4</gadget>
<output>17</output>

<result>17</result>