How to solve a proportion
- Fill in three of the four values in a ÷ b = c ÷ d.
- Leave the unknown one empty.
- Read the missing value and the cross-multiplication check.
Example
A map scale says 2 cm is 5 km. How far is 7 cm? 2/5 = 7/x, so 2x = 35 and x = 17.5 km.
Direct and inverse proportion
This calculator is for direct proportion: double one value and the other doubles. Inverse proportion works the other way: twice as many workers take half as long, so a × b stays constant instead.
Scaling a recipe
A recipe for 4 people uses 300 g of pasta. For 6 people: 300/4 = x/6, so x = 450 g. Every ingredient scales the same way, which is the whole idea of a direct proportion.