If A and B are two isotopes of an element with masses 12 and 13 respectively occurring 98.89% and 1.11% respectively, what can say about the average mass of the element?

12

We see that A occurs 98.89% in nature that means in average mass, the most contribution will be due to this very isotope. Thus, the mass should be around mass the of A which is 12. It can't be less than 12 since the other mass is 13 occurring in 1.11%. It can't be 20 either as this will to far off. Alternatively, if you want go by calculations, :

Average mass = (m_A x 98.89 + m_B x 1.11)/100