×QvantCase
← All techniques

Checking answers · level 1

Checking with digit sums

Add the digits of a number until one digit is left. Digit sums survive multiplication and addition, so they catch almost every slip you can make. Drill: give the digit sum of the number shown.

Worked example: digit sum of 4,867,295

digits4 8 6 7 2 9 5
first pass41
reduced5

Step by step

  1. 1

    Add every digit

    4 + 8 + 6 + 7 + 2 + 9 + 5 = 41

  2. 2

    Reduce to one digit

    41 → 5

  3. 3

    Shortcut

    Pair digits that make 9 and ignore them — the result is the same.

digit sum of 4,867,295 = 5

The theory behind it

Intuition

Digit sums survive multiplication and addition, so a one-line check catches most transcription errors without redoing the sum.

Common pitfalls

  • ×Digit sums cannot catch a transposed pair of digits.
  • ×Skipping the check on the number you will say out loud.

In the interview

Say the check aloud — interviewers reward candidates who sanity-check their own arithmetic.