Proportion Solver
Solve a/b = c/d
Solve a/b = c/d — leave one field blank
About This Calculator
A proportion states that two ratios are equal: a/b = c/d. Given any three of the four values, you can solve for the missing one using cross-multiplication. Proportions are used in scaling recipes, maps, unit conversions, and many real-world problem-solving scenarios.
Formula
a/b = c/d → cross-multiply: a × d = b × c
Missing value = (product of known diagonal) ÷ (remaining known value)
Example Calculation
Solve: 3/4 = x/20
- Cross-multiply: 3 × 20 = 4 × x
- 60 = 4x
- x = 60 / 4 = 15
x = 15
Proportion Examples
| a | b | c | d (solved) | Check |
|---|---|---|---|---|
| 1 | 2 | 5 | 10 | 1/2 = 5/10 |
| 3 | 4 | 9 | 12 | 3/4 = 9/12 |
| 5 | 8 | 15 | 24 | 5/8 = 15/24 |
| 2 | 7 | 6 | 21 | 2/7 = 6/21 |
| 4 | 10 | 12 | 30 | 4/10 = 12/30 |
Frequently Asked Questions
What is direct proportion?
In direct proportion, when one value increases the other increases by the same ratio. If y = kx, then y is directly proportional to x with constant k.
What is inverse proportion?
In inverse proportion, when one value doubles, the other halves. If y = k/x, then y is inversely proportional to x. Speed and travel time are a classic example.
How are proportions used in real life?
Proportions are used for scaling recipes (doubling a recipe), map reading (1 cm = 10 km), currency conversion, and mixing solutions to a specific concentration.