What is the rule of three?

A method to find a missing value in a proportion: if A corresponds to B, then C corresponds to X. In direct proportion X = B × C ÷ A.

When should I use inverse proportion?

When increasing one quantity decreases the other, such as workers and days to finish a job. Then X = A × B ÷ C.

If 3 notebooks cost $12, then 5 notebooks cost 12 × 5 ÷ 3 = $20 (direct). If 3 workers take 12 days, 6 workers take 3 × 12 ÷ 6 = 6 days (inverse).

The division is undefined, so the result shows a dash. Nothing you enter leaves your browser.

Solve direct and inverse proportions from three known values. Nothing leaves your browser.

If A gives B, then C gives X: A/B = C/X.

What is the rule of three?: A method to find a missing value in a proportion: if A corresponds to B, then C corresponds to X. In direct proportion X = B × C ÷ A.
When should I use inverse proportion?: When increasing one quantity decreases the other, such as workers and days to finish a job. Then X = A × B ÷ C.
Can you give an example?: If 3 notebooks cost $12, then 5 notebooks cost 12 × 5 ÷ 3 = $20 (direct). If 3 workers take 12 days, 6 workers take 3 × 12 ÷ 6 = 6 days (inverse).
What if A or C is zero?: The division is undefined, so the result shows a dash. Nothing you enter leaves your browser.