How this calculator works
A proportion states that two fractions have the same value. Cross multiplication turns a/b = c/x into ax = bc. Choose the missing term and rearrange that equality. Known denominators, divisors used in solving, and solved denominators must be non-zero.
Formula
a/b = c/x means ax = bc. Solve x = bc/a, a = bc/x, b = ax/c, or c = ax/b.
Worked example
For 2/5 = 6/x, cross multiplication gives 2x = 30, so x = 15. If the left numerator is missing in a/5 = 6/15, a = 2.
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Frequently asked questions
Why choose the missing term instead of leaving it blank?
The selection identifies which field to solve. Keep a valid number in every visible field; the selected unknown is replaced by the calculated answer.
What happens if zero makes many answers possible?
A zero multiplier can make the equation impossible or leave infinitely many solutions. This calculator requires one defined answer and reports an error in those cases.
Can the known values be decimals?
Yes. The solve modes accept finite decimal values; only a zero in a divisor position is rejected.