We know that combined wave function is given as:
Ψ AB = Ψcovalent + λΨionic
The percent ionic character is given by:
[λ2/(1 + λ2)] X 100
Therefore, 20 = 100*[λ2/(1 + λ2)]
20 + 20 λ2 = 100λ2
20 = 100λ2 – 20λ2
20 = 80λ2
Therefore, λ2 = 20/80
Hence, λ=(20/80) 1/2= 0.5
Thus, the fraction of contribution of ionic character to Ψ AB is 0.5