超级大脑之多位数相减

brain training, mental arithmetic, column calculation, puzzle challenge 4. Description: This app trains borrowing, mental subtraction stability, and focus. Each issue shows two numbers with the same digit count. The player subtracts by columns and enters the complete non-negative difference. The generated issue set has three tiers: easy uses 2-digit subtraction, medium uses 3-digit subtraction, and hard uses 4-digit subtraction. The generator always keeps the minuend greater than or equal to the subtrahend, so every issue is solvable and non-negative. Answers keep a fixed length per tier and use leading zeroes when needed. Short intro: subtract from right to left, handle borrowing, and enter the correct difference.