You have a bag of cotton and an iron bar, each indicating a mass of 100 kg when measured on a weighing machine. In reality, one is heavier than other. Can you say which one is heavier and why?
We know that Density of cotton is less than Density of iron.
And Density = Mass/Volume
So, that means Volume of 100 kg Cotton is much more than volume of 100 kg iron
Now, the air also applies a force in the upward direction due to buoyancy (gases also exert buoyant force)
Since cotton has larger volume, the upward buoyant force (upthrust) on cotton is much more than that of iron.
Hence, weighing machine will show much less mass of cotton than it actually is.
Therefore, 100 kg cotton is more heavier than 100 kg iron