Proportion calculator (rule of three)
If A corresponds to B, what corresponds to C? Choose whether the proportion is direct or inverse.
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How it is calculated
Direct: X = B × C / A
Inverse: X = A × B / C
A proportion is direct when one quantity grows in step with the other (more pounds, higher price) and inverse when one grows as the other shrinks (more workers, fewer days).
Frequently asked questions
Direct proportion example
If 2 lb cost $6, 5 lb cost 5 × 6 / 2 = $15.
Inverse proportion example
If 4 workers take 6 days, 3 workers take 4 × 6 / 3 = 8 days.
How do I know if it is direct or inverse?
Ask what happens if you double A: if B doubles too, it is direct; if B halves, it is inverse.