Note that there are two different types of B atom: one type has 5p, 5e, and 5 n, while the other type has an extra neutron (5p, 5e, 6n). These are ISOTOPES of boron, identical in every way except in the number of neutrons.
Atoms 1, 5, and 6 have 5p, 5n, and 5e. Since protons and neutrons both have a mass of 1 amu (atomic mass unit), then the atomic mass of these atoms is the sum of the masses of the protons and neutrons, which is 5 amu + 5 amu = 10 amu. We call this isotope B-10. Note that we did not consider the mass of the 5 electrons since they are so small. The rest of the atoms contain 5p, 5e, and 6n. The atomic mass of these atoms = 5 amu + 6 amu = 11 amu so we call them B-11.
So far, we have three B-10 atoms and seven B-11 atoms.
Suppose you want the AVERAGE atomic mass for all ten B atoms. Let's calculate that.
.3 B-10 atoms x 10 amu / atom = 30 amu
+7 B-11 atoms x 11 amu / atom = 77 amu
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10 B-10/B-11 atoms have a total mass = 107 amu.
If 10 B atoms have a mass of 107 amu, then 1 B atom has a mass = 107 amu / 10 atoms = 10.7 amu. This is the average atomic mass for all ten B atoms. Note how that compares to the average atomic mass for B given on the periodic table (10.81 amu) ... pretty close.
I hope that this helped.