Final answer:
The missing digits A, B, and C in the addition problem are 8, 0, and 6, respectively, determined through column-wise addition and carry-over.
Step-by-step explanation:
The student is asked to determine the missing digits, A, B, and C, in a column addition problem. We can solve this by starting from the rightmost column and moving left while considering any carry-over values. Adding the ones column gives us 1 in the subtotal, so C must be 6 since 1 + 4 is 5, and we need a total of 11 in that column with the additional 6. Moving to the tenths column, we already have a carry-over of 1, adding to 9 and 2 gives us 12, so we put down 2 and carry over another 1. For the hundredths column, we add 1 (our carry-over), 3 from 25.31, and A from 21.2A. The subtotal here must be 12 because of the 2 below, so A is 8. Lastly, we add 1 (the last carry-over), 5, and 2 to get B, which needs to total 7. So, B is 0. The final numbers are A = 8, B = 0, and C = 6.