Divisibility rule:
The number is divisible by 9 if the sum of the digits of the number is divisible by 9.
5 + 3 + 2 + 1 = 11
18 is divisible by 9, to find blank, simply subtract the numbers:
18 - 11 = 7
Check:
53721 ÷ 9 = 5969
Therefore, the answer to your question is 7.
