Naturally occurring boron consists of two isotopes whose atomic weighs are 10.01 and 11.01. The atomic weight of natural oron is 10.81. Calculate the percentage of each isotope in natural boron.

Igniting MnO₂ converts it quantitatively to Mn₃O₄. A sample of pyrolusite is of the following composition : MnO₂ 80%, SiO₂ and other inert constituents 15%, rest being water. The sample is ignited in air to constant weight. What is the percentage of Mn in the ignited sample? [O = 16, Mn = 54.9]