Explanation:
All boron isotopes have 5 protons.
B11 has 6 neutrons, so its atomic mass is:
6 (1.008664 u) + 5 (1.007276 u) = 11.0884 u
B10 has 5 neutrons, so its atomic mass is:
5 (1.008664 u) + 5 (1.007276 u) = 10.0797 u
So the average atomic mass of boron is:
0.80 (11.0884 u) + 0.20 (10.0797 u) = 10.8866 u
The closest whole number is 11.