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The reading of barometer gives the pressure in cm of Hg

Given

Reading of barometer on a day= atmospheric pressure of that day P= 76cm of Hg=0.76m of Hg

Drop in pressure when water is introduced P’=75.4cm of Hg

=0.754m of Hg

So, vapor pressure =P-P’=0.76-0.754=0.006m of Hg

Given saturation vapor pressure = 1cm of Hg=0.01m of Hg

So

the relative humidity in the space above the mercury column is 60%.