Lab · native contrast

Rebuild Calculator’s % button — try both rules

Why does 200 + 10% become 220, while 200 × 10% becomes 20? The percent key is operator-dependent. Microsoft publishes Calculator sources, so you can arbitrate REA findings against named symbols. This page hosts a small dual-rule trial for operators on rea.run.

With +: Take 10% of the first number.
10% of 200 = 20 → 200 + 20 = 220
With ×: Turn 10% into 0.1.
200 × 0.1 = 20

Readable rule (teaching summary)

if (operation == multiply || operation == divide) {
  percent = current / 100;
} else {
  percent = current * previous / 100;
}

For previous = 200, current = 10: add path yields 20 then 220; multiply path yields 0.1 then 20.

Open-source arbitration

Item	Value / link
Public tree	microsoft/calculator (MIT)
Handler (pinned commit)	scifunc.cpp · IDC_PERCENT
Unit tests (pinned)	CalculatorManagerTest.cpp
Command IDs	`IDC_MUL 92 (0x5c)`, `IDC_DIV 91 (0x5b)`, `IDC_PERCENT 118 (0x76)`; constant `0x64 = 100`

case IDC_PERCENT:
{
  // multiply/divide → X [op] (Y%)
  // otherwise → X [op] (X * Y%)
  if (m_nOpCode == IDC_MUL || m_nOpCode == IDC_DIV)
    result = rat / 100;
  else
    result = rat * (m_lastVal / 100);
  break;
}

Verifiable trial matrix

Expression	Expected %	Expected result	Check on this page
200 + 10%	20	220	Select +, press %, press =
200 × 10%	0.1	20	Select ×, press %, press =

Operator steps

Read the short showcase summary: /showcase/windows-calculator-percent/.
Open the pinned scifunc.cpp and tests above; name the branch before trusting a screenshot.
If you inspect an installed Calculator binary with REA, keep Evidence + public source side by side.
Rebuild only teaching demos you control — not advice to tamper with OS components you are not authorized to modify.

Use REA to inspect Windows Calculator’s % button.
Recover the rules after + and ×, then build a small calculator that uses both.

Teaching disclosures

This widget is an interactive illustration of the inspected percentage rule — not the Windows Calculator UI.
Prefer Microsoft’s public MIT tree as the arbitration layer when claims conflict.
Authorized / educational / interoperability study only. See /compliance/ and /faq/legal-boundary/.

Resource	Link
Lab hub	/lab/
Dino speed lab	/lab/dino-speed/
Showcase summary	/showcase/windows-calculator-percent/
DX-Ball Evidence	/showcase/dx-ball/
Chinese twin	/zh/lab/calculator-percent/