The 84% Method: A Terminal-Digit Selection Heuristic for Two-Digit Mental Multiplication

The 84% Method is a selection heuristic for two-digit mental multiplication. It classifies a factor as favorable when its units digit is 0, 1, 2, 5, 8, or 9. Under uniformly distributed terminal digits, the probability that at least one of two factors is favorable is 1 − (4/10)² = 84%. The 84% figure refers to the coverage of the selection criterion, not to a measured speed improvement.

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Publication Details

Journal
Zenodo (CERN European Organization for Nuclear Research)
Published
2026-09-05
DOI
https://doi.org/10.5281/zenodo.22345462
Primary Topic
Cognitive and developmental aspects of mathematical skills
Type
preprint
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preprint

The 84% Method: A Terminal-Digit Selection Heuristic for Two-Digit Mental Multiplication

Timon Felice Amand Roder
Zenodo (CERN European Organization for Nuclear Research)
Cognitive and developmental aspects of mathematical skills
preprint

The 84% Method: A Terminal-Digit Selection Heuristic for Two-Digit Mental Multiplication

Timon Felice Amand Roder
preprint en

Abstract

The 84% Method is a selection heuristic for two-digit mental multiplication. It classifies a factor as favorable when its units digit is 0, 1, 2, 5, 8, or 9. Under uniformly distributed terminal digits, the probability that at least one of two factors is favorable is 1 − (4/10)² = 84%. The 84% figure refers to the coverage of the selection criterion, not to a measured speed improvement.

Zenodo (CERN European Organization for Nuclear Research)
University of Tübingen (DE)
Cognitive and developmental aspects of mathematical skills
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