Proportional Calculator Pro.

This is the 2nd of the 'BEST TOOLS' series with the motto of 'Math that works'. Here is a practical way of the use of the Rule of Three, once you start using it, you will discover how useful it can be in the WORK and the EDUCATIONAL areas.

What makes this app very useful is the ability to create your own equivalences and name the units at your own way, giving you the option to save them so you can recall them any time you need them.

Another point is the easy way to create new equivalences.
To avoid confusions you can see at the top a logic sentence that describe the numerical values interactively way while your are typing them (Right way to read the Rule of Three).
...(This is the beauty of Math when it is applied to daily live problems.)

An example could be in the calculation of percentage: let say that if 23 boxes are the 100% then 6 boxes what percentage will be? just write the numbers and your will get the result. There are endless of applications just experiment by yourself.