Answer is: mass of the other isotope of boron is 10.01 amu.
Ar₁(B) = 11.01 amu; the average atomic mass of isotope ¹¹B.
Ar₂(B) =?; the average atomic mass of unknown isotope of boron.
ω₁(B) = 80.00%; mass percentage of isotope ¹¹B.
ω₂(B) = 100.0% - 80.00%.
ω₂(B) = 20.00%.; mass percentage of unknown isotope of boron.
ω₂(B) = 20.00% ÷ 100% = 0.20.
Ar(B) = 10.81 amu; average atomic mass of boron.
Ar(B) = Ar₁(B) · ω₁(B) + Ar₂(B) · ω₂(B).
10.81 amu = 11.01 amu · 0.8 + Ar₂(B) · 0.2.
Ar₂(B) = 2.002 amu ÷ 0.2.
Ar₂(B) = 10.01 amu.
