Start at the right column, and think about what two different numbers
add up to 4. 1 and 3 fit that description, so plug them in for A
and B. You see now that that can’t work because
13 + 31 = 44, a 2-digit number. The answer to this addition
problem has 3 digits (CD4). Now go back to the right column: think
of 2 different numbers that add up to 14. The only options are 6
and 8, and 9 and 5. 9 and 5 don’t work because if you complete the
addition problem, D would be 5, and all the letters represent different
numbers.