The rule of three: a practical guide
Solve direct proportions by arranging equivalent ratios, cross-multiplying and checking units.
The rule of three finds an unknown value in a direct proportion. If a corresponds to b, and c corresponds to x:
a / b = c / x
x = b x c / aIf 3 notebooks cost $12, then 5 notebooks cost:
3 / 12 = 5 / x
x = 12 x 5 / 3 = 20The result is $20.
Arrange like units in the same positions before cross-multiplying. Then check whether the relationship is direct: when one quantity increases, should the other increase too? Fuel used over a fixed efficiency is direct; workers and time for a fixed job may be inverse and require a different setup.
Finally, inspect the units and estimate the scale. Five notebooks should cost more than three, so a result below $12 would signal an error.
Use the rule of three calculator to solve and verify a direct proportion.