Proportion Solver (A : B = C : x)
Simplify ratios, solve proportions A : B = C : x, scale ratios by any factor and convert them to fractions, decimals and percentages — with every step shown.
Result
This page opens the ratio calculator in proportion mode, preconfigured to solve A : B = C : x. Type the three known values and the missing fourth is found by cross-multiplication: for 3 : 4 = 9 : x the calculator computes x = (4 × 9) ÷ 3 = 12.
The working is written out in full — the cross-multiplication line followed by the division — using exact integer arithmetic. When the answer is not a whole number you get the exact fraction in lowest terms together with its decimal value, so nothing is silently rounded.
Proportions are the universal tool for scaling: converting recipe quantities to a different number of guests, reading distances off a map scale, mixing paint or fertilizer at a fixed strength, or estimating a price for a different amount. Decimals such as 2.5 : 1 = 10 : x work exactly the same way.