prop-cli 1.0.1

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

prop-cli

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

a : b = c : d

where one of the terms is unknown (x).
This is the classic rule of three, a fundamental arithmetic technique for solving proportions by cross-multiplication.

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Installation

Clone the repository and install locally:

pip install .

This will install the prop-cli command.

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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 put x in any of the four positions:

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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How cross-multiplication works

To solve a proportion of the form:

a : b = c : d

we use cross-multiplication, which states that:

a × d = b × c

This is the fundamental property of proportions.

Visual ASCII diagram

   a : b = c : d
     │       │
     │       │
     └───┬───┘
         │
 Cross-multiplication
         │
   a × d = b × c

Solving for an unknown (example)

If one term is unknown (x), we isolate it using the cross-multiplication identity.

Case: the unknown is in the second position (x):

    a : x = c : d
      │       │
      │       │
      └───┬───┘
          │
  a × d = x × c
          │
          └──→  x = (a × d) / c

Another case: unknown in the fourth position (x):

    a : b = c : x
      │       │
      │       │
      └───┬───┘
          │
  a × x = b × c
          │
          └──→  x = (b × c) / a

This tool automatically:

• identifies where the x is,
• applies the correct formula,
• prints the solved proportion and the computed value.

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Error handling

The tool validates:

• only one unknown term (x)
• all other terms must be valid numbers
• division by zero is caught with clear error messages

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Project structure

prop-cli/
├── pyproject.toml
├── README.md
└── proportion_calculator/
    ├── cli.py
    └── __init__.py

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License

MIT License