Proportion Solver
Solve a/b = c/x for the missing fourth value — the exact structure behind recipe scaling, map-scale distances, and unit-price comparisons, without cross-multiplying by hand.
Inputs
Result
x = 20
How it works
- 13/4 = 15/x
- 2x = 20
How the proportion solver works
Cross-multiplying a/b = c/x gives x = (b × c) ÷ a.
Worked example: 3/4 = 15/x
- x = (4 × 15) ÷ 3 = 60 ÷ 3 = 20.
- Check: 3/4 = 0.75 and 15/20 = 0.75 — the two ratios match, confirming x = 20 is correct.
Common mistakes to avoid
Setting up the proportion with mismatched units on each side
a/b and c/x need to represent the same kind of relationship in the same order — e.g. if a/b is miles-per-hour, c/x must also be miles-per-hour, not hours-per-mile, or the answer will be inverted.
Entering a = 0
Since x = (b×c) ÷ a, an a of 0 makes the division undefined — there's no proportion to solve if the first ratio's numerator is zero.
Frequently asked questions
What's a real example this solves?
If 3 kg of a mixture costs 15 dollars, how much do you pay for 20 kg? Set up 3/15 = 20/x and solve for x, the cost.
Can this solve for a different position, like the 'b' value instead of x?
The tool is set up specifically to solve for the fourth term given the other three in the a/b = c/x layout — rearrange your known values into that position before entering them.
How is this different from the Ratio Calculator?
The Ratio Calculator simplifies a single given ratio; this tool solves for an unknown fourth value across two equal ratios.
Does the direction of the ratio matter?
Yes — a/b = c/x is a different equation from b/a = c/x, so make sure both ratios are set up in the same order (e.g. both as 'part over whole' or both as 'whole over part').
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