It uses a practical process to solve problems involving four values ​​of which we know three of them.  Consider four practical steps used in one of three simple rules:   Building a table, gathering the quantities in columns and relating each value his greatness;  Identify if the quantities are directly or inversely proportional, which we will see after the last step;  Assemble the equation as follows: if the quantities are directly proportional, the multiply values   X-shaped; if the quantiti