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 quantities are inversely proportional, the multiplies in line values;

 Solving the equation.