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Addition & subtraction · level 1

Left-to-right addition

Adding from the left keeps a single running total and gives you the leading digits first — which is what you usually care about. Add the thousands, then hundreds, then tens, then units.

Worked example: 4,652 + 3,789

+ 3,000
+ 700
+ 80
+ 9
total8,441

Step by step

  1. 1

    Start from the first number

    4,652

  2. 2

    Add 3,000

    4,652 + 3,000 = 7,652

  3. 3

    Add 700

    7,652 + 700 = 8,352

  4. 4

    Add 80

    8,352 + 80 = 8,432

  5. 5

    Add 9

    8,432 + 9 = 8,441

4,652 + 3,789 = 8,441

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.