total pressure = 10

partial pressure of oxygen = 20/100 x 10 = 2 atm = 2 x 760mm

partial pressure of nitrogen = 79/100 x 10 = 7.9 atm = 7.9 x 760mm

(since constants are given in mm convert it into mm)

then now apply henry's law and find  mole fraction of each....dat is the required ans