The Combined Gas Laws: The Gas Laws Part 3--- Science Blog Post

Jacques Charles discovered that the relationship between a gas's volume is directly proportional to its temperature in Kelvins when the pressure and number of particles stay the same.  Robert Boyle discovered that the relationship between a gas's volume is inversely proportional to the pressure of the gas when the temperature and number of particles are constant.  But, a third law is created when you combine these two laws: The Combined Gas Law.
The equation for the Combined Gas Law combines the equations for Charles's Law and Boyle's Law into one formula.

      

Basically, you take the Boyle's Law equation and place it over the denominator for the Charles's Law.  It is very simple.  This law can be used when you need to find three changing variables: Temperature, pressure, and volume. You will have six quantities, one of which will be unknown.

For more information, read:

    Wysession, Michael, David V. Frank, and Sophia Yancopoulos. Prentice Hall Physical Science: Concepts in Action. Needham, MA: Pearson/Prentice Hall, 2004. Print.