E= set of numbers divisible by 3 E={3,6,9,12,.........1998} ∴n(E)=666 F= set of numbers divisible by 7 F={7,14,21,.........1995} ∴n(F)=285 E∩F= set of numbers divisible by 3 and 7 ={21,42,.........1995} ∴n(E∪F)=606+285−95=856 Required probability =