The total pressure in a flask containing air and ethanol at 257C is 900 mm Hg. If the pressure of the air in the flask is 762 mm Hg, the vapor pressure of ethanol is (blank) mm Hg.
Dalton's Law of partial pressure says that
total pressure is the sum of the partial pressure of each gas.
Ptotal = Pair + Pethanol.
You know total, you know air, calculate pressure ethanol.