Boron obtain from borax deposits in Death Valley consists of two isotopes. They are boron-10 and boron-11...?

This is easy. Since the average atomic m of 10.81 is much closer to the m of the heavier isotope (11.009), that isotope must be more abundant. (If the two isotopes had been present in equal quantities, the average m would have been halfway between, or 10.511.)