One possible source of confusion is this.  If the world were always in equilibrium, then the air would ALWAYS be saturated with water vapor.  Fortunately, the world is usually not in equilibrium.  The air around us has cycled through other conditions and lost much of the water vapor that it would be carrying if it were in thermodynamic equilibrium with the lakes and oceans around us.  Evaporation from the lakes and oceans is always in process as the system tries to reach equilibrium, but the winds blow in nice new unsaturated air, so equilibrium is not always attained.

Relative humidity is defined to be the actual vapor pressure of water vapor in the air at the real observed temperature, divided by the saturated vapor pressure of the air at the same temperature.  The denominator is found by looking it up, on a table like the one given in problem 18.85.  The numerator is found by locating the "dew point," and then looking up the saturated vapor pressure at the dew point temperature on the same table.  The saturated vapor pressure at the dew point temperature is the actual vapor pressure at the real temperature.