---
title: "Why does 200 + 10% give 220 in Windows Calculator?"
url: "https://reamcp.dev/en/cases/why-does-200-10-give-220-in-windows-calculator-d80c96"
source: "https://rea.run/showcase/windows-calculator-percent"
source_type: "website"
author: "REA community"
author_handle: "REA community"
stars: 0
category: "CTF & learning"
categories: ["ctf-learning"]
tags: []
added: "2026-10-13T03:57:33.902Z"
updated: "2026-10-13T03:57:33.902Z"
locale: "en"
content_locale: "en"
---

# Why does 200 + 10% give 220 in Windows Calculator?

Explains the familiar percent-key quirk and checks the explanation against the public, open-source Calculator code.

## Description

Why it is listed: Checked against open-source code; educational.

## Original README

Why 200 + 10% can yield 220 — and how to check explanations against Microsoft’s public sources.