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Single Rule of Three Direct

Find the fourth value in a direct proportion from three known values, where a larger first quantity means a larger result.

About Single Rule of Three Direct

The rule of three finds an unknown fourth value from three known ones that are in proportion. In a direct proportion both quantities rise together, and X = B × C ÷ A. In an inverse proportion one falls as the other rises, and X = A × B ÷ C. For example, if 5 workers need 20 days, 8 workers need 5 × 20 ÷ 8 = 12.5 days. The divisor cannot be zero.

How to use Single Rule of Three Direct

  1. Fill in "Type of proportion", "If A", "corresponds to B" and "then C".
  2. The result appears below and updates as you type.
  3. Copy the result with the copy button, or download it as a file where that is offered.

Questions about Single Rule of Three Direct

How do I know whether a problem is direct or inverse?

Ask what happens to the answer when the first quantity grows. If more items cost more, it is direct. If more workers finish the same job in less time, it is inverse.

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