'W' g of a non-volatile electrolyte solid solute of molar mass 'M' g mol^–1 when dissolved in 100 mL water, decreases vapour pressure of water from 640 mm Hg to 600 mm Hg. If aqueous solution of the electrolyte boils at 375 K and K_b for water is 0.52 K kg mol^–1, then the mole fraction of the electrolyte solute (x₂) in the solution can be expressed as
(Given : density of water = 1 g/mL and boiling point of water = 373 K)