Xinhua Bookstore has newly arrived six boxes of books, and the number of books in them is not equal, namely: 15 books, 16 books, 18 books, 19 books, 20 books, and 31 books. Among the five boxes, the total number of books in three boxes is exactly twice the total number of books in the other two boxes. How many books are in the box other than these five boxes?
新华书店新到六箱书，装的本数都不相等，分别是：15本、16本、18本、19本、20本、31本．在其中的五个箱子中，有三个箱子的总本数刚好是另两个箱子总本数的2倍，除这五个箱子外的那个箱子装有几本书？
<gadget id="calculator">15 / 1</gadget>
<output>15</output>

<gadget id="calculator">15 + 5</gadget>
<output>20</output>

<result>20</result>