Solve for the missing value in an A:B = C:D proportion.
Free to use, no sign-up required. Proportion Solver is one of 198 tools built into Xenition, alongside a chat that can use them for you.
Solves a proportion of the form a / b = c / x for the missing value. It covers scaling recipes, map distances, unit rates and most “if this, then how much” problems.
Cross-multiplying a / b = c / x gives a × x = b × c, so x = b × c ÷ a. It shows a dash when a is 0, and the result is given to four decimal places.
2 / 5 = 8 / x gives x = 5 × 8 ÷ 2 = 20. If 2 cups of flour make 5 muffins, then 8 cups make 20.
Keep the same kind of quantity in the same position on both sides. If 3 hours cost 45, how much do 7 hours cost? Enter a = 3, b = 45, c = 7, and x = 105.
Only when the two quantities rise at a fixed rate together. It fails for things like baking time, which does not double when you double a recipe.
You do not have to open Proportion Solver to use it. Ask Xenition in plain language and it runs the right tool, then carries the answer into whatever you are working on — a document, a spreadsheet, a deck.