People’s Primary School held a reading festival activity. Xiaolin from Class 5 and 3 read a book with 400 pages in total. After reading it for 4 days, Xiaolin happened to finish reading the reverse side of a certain page. The sum of the two pages with bookmarks was 81. According to this calculation, How many days will it take for Xiao Lin to finish this book?
人民小学开展读书节活动，五3班的小林读一本书共400页，小林看了4天后，正好看完某一页书的反面，夹书签的两页之和是81，照这样计算，小林看完这本书还要几天？
<gadget id="calculator">400 - 40</gadget>
<output>360</output>

<gadget id="calculator">40 / 4</gadget>
<output>10</output>

<gadget id="calculator">360 / 10</gadget>
<output>36</output>

<result>36</result>