How do I know if it is direct or inverse?

If both quantities grow together, like quantity and price, it is direct. If one grows while the other shrinks, like workers and days, it is inverse.

Can I use it for percentages?

Yes. Put 100 in A, the total in B and the percentage in C. For percentages, the percentage calculator on this site is quicker.

Find the missing value in a proportion: if A is to B, what is C to? Works for direct and inverse proportions.

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Solve direct and inverse proportions, also known as the rule of three, with the formula and the result right away. For recipes, prices and time.

How do I know if it is direct or inverse?: If both quantities grow together, like quantity and price, it is direct. If one grows while the other shrinks, like workers and days, it is inverse.
Can I use it for percentages?: Yes. Put 100 in A, the total in B and the percentage in C. For percentages, the percentage calculator on this site is quicker.