Download of Rule of 3 2.1 for Mac was on the developer's website when we last checked. We cannot confirm if there is a free download of this app available. The software lies within Productivity Tools, more precisely Calculators.
The actual developer of this Mac application is Everton Postay Cunha. The most popular versions among Rule of 3 for Mac users are 2.1 and 1.0. This application is developed for Mac OS X 10.7 or later. Please check the Mac app with an antivirus before launch as it is downloaded from the developer's website, and we cannot ensure that it is safe.
From the developer:
A simple App to help you calculating the rule of 3!
You may want to check out more Mac applications, such as Slide Rule, 2d ruler or ScreenRuler, which might be similar to Rule of 3.
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