# Windows Calculator: reconstructing percentage rules

> REA’s case traces the installed Windows Calculator model DLL to explain different percentage behavior for multiplication/division and other operations. An analyst-selected function is interpreted against public Microsoft source and demonstrated with fixed browser examples.

- Type: Case study
- Targets: App · Binary
- Category: Official REA: case studies
- Relationship: Official REA case study
- Sources reviewed: 2026-10-17
- Added to the catalog: 2026-10-17
- Original resource: <https://github.com/morluto/rea/blob/c360cfcd6d7901514b16dd6113eba2e0b0fdf422/website/evidence/calculator-and-dino.md#windows-calculator>
- Primary source: <https://github.com/morluto/rea/blob/c360cfcd6d7901514b16dd6113eba2e0b0fdf422/website/evidence/calculator-and-dino.md>
- Catalog page: <https://awesomerea.org/resources/rea-calculator-percent-case/>
- Record JSON: <https://awesomerea.org/resources/rea-calculator-percent-case/record.json>
- Chinese version: [简体中文](https://awesomerea.org/zh/resources/rea-calculator-percent-case/index.md)

## Requirements & limits

- **Scope:** The documented target is Calculator 11.2508.4.0 x64. The public source commit is an interpretation cross-check, not an identified build commit for that installed DLL.
- **Attribution:** Function selection was analyst-assisted.
- **License:** This reconstructs a bounded arithmetic rule, not the Calculator application. Public Microsoft source is MIT-licensed; access and rights to the installed target remain separate.

## Primary sources

1. <https://github.com/morluto/rea/blob/c360cfcd6d7901514b16dd6113eba2e0b0fdf422/website/evidence/calculator-and-dino.md>
2. <https://github.com/morluto/rea/pull/1272>

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## About this entry

The description was reviewed against primary sources.
Original editorial content: MIT. Linked projects, papers and assets retain their own rights and licenses.