# Fractions Solve for Unknown Calculator

> Solve an unknown term in an equivalent-fractions proportion a/b = c/d.

Page: https://askcalculators.com/calculator/fractions-solve-for-unknown-calculator/
Definition: https://askcalculators.com/calculator/fractions-solve-for-unknown-calculator/definition.json
JSON Schema: https://askcalculators.com/calculator/fractions-solve-for-unknown-calculator/schema.json
Version: 1.0.0
Maintained by: AskCalculators (software project; not a credentialed reviewer)
Qualified review: pending. Automated checks do not establish professional approval.
Page status: Live
Review status: Pending

## Use
Open the calculator page and enter values. Query-string parameters can restore and bookmark a calculation.
Example: https://askcalculators.com/calculator/fractions-solve-for-unknown-calculator/?unknown=d&a=3&b=4&c=9&d=12

## Inputs
- unknown: Unknown term; type=select; required; default=d; values=a|b|c|d
- a: a; type=number; required; default=3
- b: b; type=number; required; default=4
- c: c; type=number; required; default=9
- d: d; type=number; required; default=12

## Outputs
- solution: Unknown value; format=number; primary
- equation: Completed proportion; format=text
- crossProducts: Cross-products check; format=text

## Formula
a/b = c/d, so ad = bc

## Rules
- Empty optional inputs are undefined, not zero.
- Hidden conditional inputs are ignored.
- Do not invent missing required inputs.