6.1. In the numerical example АБВ $+9=$ ГДЕ, the letters А, Б, В, Г, Д, and Е represent six different digits. What digit is represented by the letter Д?
Answer: 0.

Solution: In the addition, the second digit of the first addend АБВ has changed (Д instead of Б). This could only happen if 1 was carried over from the units place to the tens place during the addition. However, the first digit also changed (Г instead of А). This means that 1 was also carried over from the tens place to the hundreds place during the addition. This is only possible if Б $+1=10$. Therefore, $Б=9$, and then $Д=0$.

Note: The puzzle has solutions, for example, $194+9=203$.

Comment: The score is not reduced if the correct answer is given, it is stated that $Б=9$, $Д=0$, but detailed explanations are not provided.