prop-cli 1.0.2

A command-line tool to solve proportions using the rule of three

prop-cli

A command-line tool that solves proportions of the form:

a : b = c : d

where one term is unknown (x).
The tool uses classic cross-multiplication, also known as the rule of three.

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Installation

pip install prop-cli

This installs the command:

prop-cli

Installing from source (local clone)

If you clone this repository manually:

git clone https://github.com/Lobko/prop-cli
cd prop-cli

You can install the package locally with:

pip install .

Or, if you want to install it in development/editable mode:

pip install -e .

This allows you to modify the source code and use the prop-cli command immediately without rebuilding the package.

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Usage

prop-cli <a> <b> <c> <d>

Example:

prop-cli 10 12 x 30

Output:

Proportion: 10:12 = 25:30
x = 25

You can place x in any position:

prop-cli x 5  6  12
prop-cli 2 x  4   8
prop-cli 3 9  x  27
prop-cli 4 5 0.1  x

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More Examples

prop-cli 5 20 x 120     # x = 30
prop-cli x 4 12 48      # x = 1
prop-cli 3 x  9 15      # x = 5

Edge cases (real ones)

prop-cli 1 x 0 5        # division by zero (c = 0)
prop-cli x 0 3 2        # division by zero (b = 0)
prop-cli 1 2 3 x        # valid

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How cross-multiplication works

Given the proportion:

a : b = c : d

Rule:

a × d = b × c

Diagram (horizontal)

       a : b = c : d
         |       |
         |       |
         +---+---+
             |
   Cross-multiplication
             |
       a × d = b × c

Diagram (vertical rule-of-three)

     a      c
     ─── = ───
     b      d

Cross multiply:

a × d = b × c

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Solving when the unknown is in different positions

Unknown in second position

a : x = c : d
a × d = x × c
x = (a × d) / c

Unknown in fourth position

a : b = c : x
a × x = b × c
x = (b × c) / a

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Error handling

• Exactly one unknown x is required
• Other terms must be numeric
• Division by zero is detected and reported
• Scientific notation is avoided in output

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Project structure

prop-cli/
├── pyproject.toml
├── README.md
├── LICENSE
├── CHANGELOG.md
├── VERSION
├── tests/
│   └── test_basic.py
├── .github/
│   └── workflows/
│       ├── test.yml
│       ├── lint.yml
│       └── build.yml
└── proportion_calculator/
    ├── cli.py
    └── __init__.py

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Contributing

1. Fork the repository
2. Create a feature branch
3. Commit your changes
4. Push the branch
5. Open a Pull Request

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License

MIT License