Boyle’s law states that at constant temperature for a fixed mass, the absolute pressure and the volume of a gas are inversely proportional. In filling of air in a balloon, mass of gas (or moles) are changing as we are filling more air. so it may seem that with pressure even volume is increasing but note that even the moles are changing. therefore in P x V = constant, the value of this constant (i.e. nRT) is also increasing.
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could you please explain it again... i did't get it. moles of air filled in is changing and so is its volume. then how can P X V be constant?