Given Equation: ABB*999 = ABC123

Breaking Down the Equation:
- ABB represents a 3-digit number with the digit B repeated twice.
- When multiplied by 999, the result is a 6-digit number ABC123.

Finding the Values of A, B, and C:
1. Calculation for A:
- Since ABB is a 3-digit number, A must be 1.
- If A is any other number, the result would exceed 6 digits.

2. Calculation for B:
- To find the value of B, we need to consider the multiplication process.
- When ABB is multiplied by 999, the result will have A as the first digit, B as the second digit, and (B+1) as the third digit.
- Therefore, B must be 2. (As B+1 = 3)

3. Calculation for C:
- After determining A and B, we can find the value of C by substituting the values in the equation ABB*999 = ABC123.
- Substituting A=1, B=2, the equation becomes: 122*999 = 121878.
- This confirms that C is 8.

Therefore, the values of A, B, and C are:
- A = 1
- B = 2
- C = 8

This solution satisfies the given equation ABB*999 = ABC123, with A=1, B=2, and C=8.