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Addition & subtraction · level 2

Subtraction by complements

Round the number you are subtracting up to a friendly figure, subtract that, then give back the difference. No borrowing, ever.

Worked example: 8,000 − 3,467

rounded subtraction4,500
give back+ 33
answer4,533

Step by step

  1. 1

    Round up the number being subtracted

    3,467 → 3,500

  2. 2

    Subtract the round number

    8,000 − 3,500 = 4,500

  3. 3

    Give back the difference

    4,500 + 33 = 4,533

8,000 − 3,467 = 4,533

The theory behind it

Intuition

Adding left to right keeps the size of the number in view, so you notice an implausible total immediately. Carrying right to left hides magnitude until the end.

Common pitfalls

  • ×Reverting to school-style right-to-left under time pressure.
  • ×Dropping a zero when the columns have different lengths.

In the interview

Summing an exhibit column out loud — get the leading digits right before the pennies.