The person who can solve this problem may be smart

let me know the answer

6 the last number is the sum of rest, 2 plus 3 plus 1= 6

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Oh, i see

Another way to solve:
Organise numbers into 4 rows; Add first and third number (from the left) to get the difference between 4th and second.
Because the fourth digit is always larger than third, a sum of 3 = difference between second and fourth - 3 (second digit of fourth row) + 3 = 6.

2 + 1 = 3

3 + 3 = 6