Chlorine has two isotopes with masses 34.98 u and 36.98 u occurring in abundances of 75.4% and 24.6% respectively. What will the average mass of chlorine be?

34

Evidently, the average mass will be somewhere in between 34 and 36 which leaves us with only one option - 35. Alternatively, if you want go by calculations, :

Average mass = (34.98 x 75.4 + 36.98 x 24.6)/100