Subtract from 1000

Technique: Subtract from 1000 (or 10000) instantly

The "All from 9, last from 10" rule

To subtract a number from 1000, apply these steps to each digit:

• Subtract each digit from 9, except the last non-zero digit which you subtract from 10.

Examples (1000 − 3-digit number)

• 1000 − 357 → 9−3=6, 9−5=4, 10−7=3. Answer: 643
• 1000 − 814 → 9−8=1, 9−1=8, 10−4=6. Answer: 186
• 1000 − 290 → 9−2=7, 10−9=1, last digit is 0 so keep it. Answer: 710

Examples (10000 − 4-digit number)

• 10000 − 4637 → 9−4=5, 9−6=3, 9−3=6, 10−7=3. Answer: 5363
• 10000 − 8125 → 9−8=1, 9−1=8, 9−2=7, 10−5=5. Answer: 1875

Why does this work?

999 is all 9s, so subtracting each digit from 9 gives 999−n. Adding the extra 1 (from 1000 vs 999) is handled by subtracting the last digit from 10 instead.

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