Boyle’s Law of gases
According to Boyle’s Law of gases , the volume of a gas is inversely proportional to the pressure of the gas at fixed temperature.
V ∝ 1/P
So if the pressure of a gas changes from P1 to P2 , its volume will also change from V1 to V2.
Hence P1 V1= P2V2
Read about Avogadro’s Law