Answer 1 :
Name : Boyle's law
Variables : Pressure, volume
Constants : Temperature, moles of gas
Equation : PV = k
Name : Charles’s law
Variables : Volume, temperature
Constants : Pressure, moles of gas
Equation : V = kT
Name : Gay-Lussac's law
Variables : Pressure, temperature
Constants : Volume, moles of gas
Equation : P = kT
Name : Combined gas law
Variables : Pressure, temperature, volume
Constants : Moles of gas
Equation : 
Explanation :
Boyle's Law : It is defined as the pressure is inversely proportional to the volume of the gas at constant temperature and number of moles.

Charles' Law : It is defined as the volume is directly proportional to the temperature of the gas at constant pressure and number of moles.
Gay-Lussac's Law : It is defined as the pressure is directly proportional to the temperature of the gas at constant volume and number of moles.

Avogadro's Law : It is defined as the volume is directly proportional to the number of moles of the gas at constant pressure and temperature.

Combined gas law : It is the combination of above four laws.
It is represented as,

Answer 2 : The number of moles are assumed to be constant while using the combined gas law.