Compound rule of three
Solve compound rule of three problems with up to 4 quantities, directly or inversely proportional, with the math step by step.
How to useCompound rule of three
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Compound rule of three: what it is for
Set up the problem in a table: the name of each quantity, the values in the two situations and whether its relationship with the unknown quantity is direct or inverse. The calculator finds x and shows the expression used.
The ready-made example shows the classic case of workers, hours per day and days of work.
How to use
- Fill in the quantities and the values for both situations.
- Mark whether each one is direct or inverse.
- See the value of x and the math.
When to use it
- Solving math exercises and test questions.
- Working out deadlines and output in construction and factories.
- Checking answers to homework.
Frequently asked questions
How do I know if a quantity is direct or inverse?
Compare it with the quantity you want to find: if one goes up and the other also goes up, it is direct. If one goes up and the other goes down, it is inverse. Example: more workers finish the job in fewer days (inverse).
How is the math done?
The unknown value is multiplied by the ratio of each quantity: direct (new ÷ old) or inverse (old ÷ new). The calculator shows the expression used.
Is it really free?
Yes, 100% free. No sign-up, no subscription and no usage limits. All Tools123 tools are free and run right in your browser.