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Digit-Sum Check (Casting Out 9s) Exam favourite

Check a multiplication answer in seconds

The Rule

  1. 1Reduce each factor to a single digit by adding its digits repeatedly.
  2. 2Multiply those digits and reduce the result the same way.
  3. 3Reduce the given answer too. If the two don't match, the answer is wrong.
  4. 4If they match, the answer is very likely right (the check can't catch errors that differ by a multiple of 9).