Doubt from solutions

Moles of water = 306/18 = 17
Solute is an electrolyte.
So effective no of moles of solute= 3*1 = 3
(mf) Mole fraction of water
= 17/(17+3)= 17/20
By raoults law pressure = P° × mf
= (17/20)×20
= 17 mm Hg