From what I remember of chemistry, the amu of each element on the periodic chart is a weighted average of all the isotopes of that element.
So, as you indicated - 80.2% of Boron exists as B-11 and 19.8% of boron exists as some other isotope. In that case you can get an estimate of the amu of the unknown through simple math:
0.802*(11.01 amu) + 0.198*(X amu) = 10.81
Solving for x, you get 10.00 amu