Solve a proportion

“200g for 300 yen — how much for 500g?” Cross-multiplication answers it: fill three of the four positions.

Tools used in this guide

Proportion solver
Unit price comparison
Recipe scaler

The job

a is to b as c is to x: 200g costs ¥300, so 500g costs x. Cross-multiply (x = b×c÷a) and you get ¥750 — the same move behind recipe scaling, map distances, and currency estimates.

Which blank goes where

Keep each side of the equation the same kind of thing: grams with grams, yen with yen. A common slip is flipping one pair — 200:300 = 500:x is right, 200:500 = 300:x is the same answer written differently, but 300:200 = 500:x inverts the price ratio and is wrong.

The result panel shows the scale (c÷a) so you can sanity-check: 500g is 2.5× of 200g, so the price should be 2.5× too.

When proportion does not apply

Proportion assumes a straight-line relationship. Volume discounts, fixed fees, progressive tax brackets, and cooking times are non-linear — the equation gives an exact answer to a model that may not fit reality.