Proportion Calculator: Solve a/b = c/x
Enter a, b and c from the proportion a/b = c/x to find the missing value x.
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Formula
For the proportion a / b = c / x, cross-multiplying gives a × x = b × c, so:
x = b × c ÷ a (a cannot be zero)
The scale factor is c ÷ a.
Worked example
3/4 = 9/x: x = 4 × 9 ÷ 3 = 12, and the scale factor is 9 ÷ 3 = 3. For 2/5 = 7/x: x = 5 × 7 ÷ 2 = 17.5.
Frequently asked questions
How do you solve a proportion?
Cross-multiply: from a/b = c/x you get a × x = b × c, so x = b × c ÷ a. For 3/4 = 9/x, x = 4 × 9 ÷ 3 = 12.
What is the scale factor?
It is the number you multiply the left side by to get the right side. For 3/4 = 9/12 the factor is 9 ÷ 3 = 3, since both 3 and 4 are multiplied by 3.
Why can a not be zero?
The formula divides by a. If a were zero, a/b would be 0 and no value of x could balance it unless c were also zero.
Where are proportions used in everyday life?
Scaling recipes, converting map distances and mixing ratios. If 2 cups serve 5 people, 7 cups of the same mix serve x people: x = 5 × 7 ÷ 2 = 17.5.