What this calculator does
Solve an unknown term in an equivalent-fractions proportion a/b = c/d.
How it works
The calculator evaluates a/b = c/d, so ad = bc. Inputs are checked before calculation, and invalid values such as zero denominators or out-of-range selections produce a clear error instead of a misleading result.
Input query strings and outputs
Use an input name in this page’s URL as ?name=value, and join additional inputs with &. Portable shared links may instead use the compact ?ac= state parameter.
Input query strings
5-
?unknown=Unknown termChoice · Required · Default: d · Accepted values: a, b, c, d
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?a=aNumber · Required · Default: 3
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?b=bNumber · Required · Default: 4
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?c=cNumber · Required · Default: 9
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?d=dNumber · Required · Default: 12
Outputs
3-
solutionUnknown valueNumber · Primary output
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equationCompleted proportionText
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crossProductsCross-products checkText
Inputs and results
The inputs are Unknown term, a, b, c, d. Results include Unknown value, Completed proportion, Cross-products check.