Solve proportions with one unknown in a/b = c/d form. Put x in any one box and see the cross multiplication steps.
For x/12 = 3/4, cross multiply to get 4x = 36. Divide by 4, so x = 9.
The proportion is solved by cross multiplication.
- Start with x/12 = 3/4.
- Cross multiply: 4x = 36.
- Divide by 4: x = 9.
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How to Use the Proportion Calculator
Enter a value for three parts of the proportion and type x for the missing part. The calculator cross multiplies and isolates x.
- Fill in a, b, c, and d in a/b = c/d form.
- Use x in exactly one position.
- Review the cross product and final check.
Cross Multiplication Formula
In a true proportion, the product of the extremes equals the product of the means. For a/b = c/d, multiply a by d and b by c.
Solving for Any Unknown
The unknown can be in the numerator or denominator of either fraction. The equation changes slightly, but the same cross multiplication idea is used.
- If x/b = c/d, then x = bc/d.
- If a/x = c/d, then x = ad/c.
- If a/b = x/d, then x = ad/b.
- If a/b = c/x, then x = bc/a.
Proportion Examples
x/12 = 3/4
Cross multiply to get 4x = 36, so x = 9.
5/8 = x/24
Cross multiply to get 120 = 8x, so x = 15.
7/x = 21/30
Cross multiply to get 210 = 21x, so x = 10.
Common Proportion Mistakes
Putting x in more than one box
This calculator solves one unknown at a time. Use x in exactly one position.
Using zero in a denominator
Denominators cannot be zero because division by zero is undefined.
Cross multiplying in the wrong direction
For a/b = c/d, the cross products are a × d and b × c.
Proportion Calculator FAQs
Can this solve decimal proportions?
Yes. Decimal values such as 2.5 or 0.75 are accepted.
Can x be in a denominator?
Yes. Enter x in b or d, and the calculator will solve the denominator position.
What is cross multiplication?
Cross multiplication is the method of multiplying diagonal terms in a proportion: a × d and b × c.
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Reviewer
This educational math content is reviewed under the SolverCalc reviewer rule by Prof. Dr. Khalil Mudassar. WordPress adds the author bio automatically, so this page does not include a duplicate author box.
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Disclaimer
This calculator is intended for general educational and practical use. Confirm your proportion setup before relying on the answer in graded or professional work.