If 21520 books need to be placed in boxes and each box can contain at most 9 books. How many books will be in the last unfilled box? Select.
A) 9 books
B) 8 books
C) 1 book
D) 4 books
E) 5 books
The number of books that can be placed in boxes of 9 books is the first number that is divisible by 9 that occurs before 21520.   In order to divide the sum in 9 parts, the amount must be divisible by 9   Divisibility rule of 9: The sum of the digits must be divisible by 9   Sum of digits of 21520 = 10 and 9 is divisible by 9.   Hence, we need to remove 1 to this number for it to be divisible by 9   1 bookorrect Option: 1 book
<result>C</result>
If 21520 books need to be placed in boxes and each box can contain at most 9 books. How many books will be in the last unfilled box?