The quantity ratio of A and B bookshelves is 4:1. If 13 books are taken from A bookshelf and put into B bookshelves, the quantity ratio of A and B bookshelves becomes 7:5. What is the sum of the two bookshelves?
甲、乙两书架的数量比是4：1，如果从甲书架取出13本书放入乙书架，甲、乙两书架的数量比变为7：5，那么两书架的数量总和是多少本．
<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">7 / 12</gadget>
<output>7/12 = around 0.583333</output>

<gadget id="calculator">(4/5) - (7/12)</gadget>
<output>13/60 = around 0.216667</output>

<gadget id="calculator">13 / (13/60)</gadget>
<output>60</output>

<result>60</result>