Ratio and proportion solver
Solve a ÷ b = c ÷ x for the missing value.
Enter your values and press Calculate. Your inputs stay in the address, so you can switch language or share the link.
Formula
a ÷ b = c ÷ x → x = b × c ÷ a
How it works
In a proportion the cross products are equal: a × x = b × c, so x = b × c ÷ a. The scale factor c ÷ a tells you how much everything is multiplied by.
Worked example
The same calculation with the default values:
- First term (a) 2
- Second term (b) 5
- Third term (c) 8
- x = 5 × 8 ÷ 2 = 20
- 8 ÷ 2 = 4
Result: Missing value (x): 20 · Scale factor (c ÷ a): 4
Assumes a direct (linear) proportional relationship between the two ratios - this does not apply to inverse or non-linear relationships (e.g. speed and time for a fixed distance, which are inversely, not directly, proportional).
Common mistakes
- Setting up the cross-multiplication with mismatched units on each side (e.g. a ÷ b in hours but c ÷ x in minutes) - both ratios must compare the same kinds of quantities in the same units.
- Mixing up which value is the unknown - swapping x with one of the known values produces its reciprocal, not the intended answer.
- Applying a direct-proportion solution to a situation that is actually inversely proportional (more workers finishing a job faster, not slower) - this calculator only solves the direct case a÷b = c÷x.
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