For the Gas Laws, Avogadro's Hypothesis and the Combined Gas Law, the balloon (container for the gas) is changing some how. Therefore, there are two conditions; one before and one after change. For each condition, you will have parameters/variables (P,V,T,n).
What happens if you are simply looking at a gas in a balloon (the balloon goes not change). Therefore, there is only one condition. So none of the equation we have been using are valid. The new equation called the Ideal Gas Equation should be used. It is PV=nRT where P is pressure of gas, V is the gas volume, T is temperature of gas, n is the number of moles of the gas, and R is called the Ideal Gas Constant which for our purposes will be R = 0.0821 l-atm /(mole-K).
You approach these questions the same as any other gas equation. The one exception is the units. It is very important to understand that since you have the gas constant R in your equation, all other variable/parameters must have the units of the gas constant (i.e. the pressure must be in atm so if it is not you must use your conversion factors to get it into atm).