Answer:
The mystery number is 351.
Step-by-step explanation:
Let the numbers be xyz.
Since the digits total is 9, then '9' cannot be among since summing it with other digit(s) will be greater than 9. Hence, '9' cannot be among the digits.
Also, '7' cannot be among the three digit number, since adding 7 to any two of the remaining three digits (that is, 1, 3, and / or 5) will make the total greater than 9. Hence, '7' also cannot be among the digits.
The remaining digits are 1,3 and 5; which all sum up to 9. That 1 + 3 + 5 = 9.
Hence the mystery number is 351.