Given,
Room temperature, T = 16°C
Dew point temperature, T0 = 7.4°C
SVP at 7°C = 7.5mm of Hg
SVP at 8°C = 8mm of Hg
SVP at 16°C = 13.5mm of Hg
For a small change in temperature, it can be assumed that saturated vapour pressure changes linearly.
The saturated vapour pressure increases by 0.5 mm for one degree rise in temperature.
Therefore increase in saturated vapour pressure for 0.4°C increase in temperature will be 0.2mm of mercury.
Hence, the saturated vapour pressure at dew point i.e. 7.4°C is 7.7 mm of mercury.
Now the relative humidity is,