Proportion Calculator
Solve for the unknown value x in a proportion a/b = c/x using cross-multiplication. Shows all steps including verification.
About this Calculator
Solve for the unknown value x in a proportion a/b = c/x using cross-multiplication. Shows all steps including verification.
Formula & Calculations
Formula
If a/b = c/x, then x = (b × c) / a (via cross-multiplication: a × x = b × c)Where:
- a, b, c=The three known values in the proportion a/b = c/x
- x=The unknown value to solve for
Assumptions
- a must not be zero (to avoid division by zero).
- All values are real numbers.
- Cross-multiplication is valid because both sides are equal ratios.
Calculation Examples
Example 1
Cross-multiply: 3x = 4×9 = 36, then x = 36/3 = 12. Check: 3/4 = 0.75, 9/12 = 0.75.
Example 2
Cross-multiply: 2×20 = 5x → 40 = 5x → x = 8.
Frequently Asked Questions
What is cross-multiplication?
Cross-multiplication converts a proportion a/b = c/d into the equation a×d = b×c by multiplying both sides by b×d. This eliminates the fractions and makes the unknown easy to isolate.
Where are proportions used in real life?
Proportions are used in scaling recipes, map reading (scale ratios), similar triangles in geometry, currency conversion, dosage calculations in medicine, and solving for unknown quantities in many scientific fields.