Overview
A proportion is an equality of two ratios. If 2/3 = 8/x, cross multiplication gives 2x = 24, so x = 12. Enter the three known parts to solve for the fourth.
Formula
a/b = c/x ⇒ a × x = b × c ⇒ x = (b × c) ÷ a.
How it works
1
Enter a, b and c
The three known parts.
2
Cross multiply
a × x = b × c.
3
Divide by a
This gives the unknown value x.
Worked examples
Example 1: 2/3 = 8/x
x = (3 × 8) ÷ 2 = 12.
Example 2: 5/10 = 3/x
x = (10 × 3) ÷ 5 = 6.
Frequently asked questions
Can I use decimals?
Yes, and the solution follows the same formula.
What if a is zero?
The proportion cannot be solved with this formula because division by zero is undefined.
How do I verify the answer?
Cross multiply; both sides should be equal.
This calculator is for educational and informational purposes only and is not a substitute for professional advice.