Mental subtraction
Borrowing across columns is where mental subtraction falls apart. The fix is to stop borrowing: count up from the smaller number, or round the ugly one and correct.
These three methods keep every step small enough to hold in working memory — the real secret of mental subtraction.
Method 1 · Level 1
Counting Up: Subtract by Measuring the Gap
645 − 297: count up 297 → 300 (3) → 600 (300) → 645 (45). The gap is 3 + 300 + 45 = 348.
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Method 2 · Level 2Subtracting Numbers Near Round Bases
823 − 489: subtract 500 instead (323), then give back the 11 you over-subtracted: 323 + 11 = 334.
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Method 3 · Level 2Left-to-Right Subtraction (No Borrowing)
845 − 623: 800 − 600 = 200, 40 − 20 = 20, 5 − 3 = 2 → 222.
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