There are two major isotopes of Lithium. The first has a mass of 6.015 amu and represents 7.59%. The other is mass 7.0160 amu, which accounts for 92.41%. What is the
average atomic mass of naturally occurring Lithium?

6.9399amu

Explanation:

Isotopes are elements with same atomic number but different atomic masses.

To determine the average atomic mass one must multiply the absolute atomic mass (amu) of each isotope times its absolute atomic mass. This will give the weight average contribution of each isotope. The sum of the weight average contributions gives the final weight average atomic mass of the element. The wt. avg. atomic mass is the value posted on the periodic table.

It is recommended that a table be sketched in the following form ...

Isotope  |    %             fractional amt. X Isotopic mass(amu) => Wt. Avg**.

   Li-6       7.59%    =>      0.0759       x         6.015amu        =>     0.4565

   Li-7      92.41%    =>      0.9241        x         7.016amu        =>     6.4835

*amu => atomic mass units                                 ∑ Li-6 + Li-7  =   6.9399amu

**Wt.Avg. => weight average contribution of isotope = fractional amt x Isotopic mass(amu)

*** Final Wt. Avg. = ∑Wt.Avg. Contributions (value posted of periodic table)

the answer is b. dilution