Answer :
Explanation :

From the assumption and trial and error method, last three digits sum must leave the same digit as the answer. It is possible only if the answer must be units digit number leaving tens digit as carry. And the possible 3rd digit is 5. Since 5 + 5 + 5 = 15  Now, 2nd step is  1 + x + x + x = 5 => 3x = 4 It is possible with x = 8. 8 + 8 + 8 + 1 = 25 leaving 2 as carry.  Now, 3rd step is 2 + x + x + x = 5 => x = 1   Hence, the three dgits are 1, 8, 5. 185 + 185 + 185 = 555.