The number of moles of a substance is given by:
(mass (grams))/("Relative atomic mass", A_r)mass(grams)Relative atomic mass,Ar
And one mole comprises (Avogadro's constant), N_A =6.022 times 10^23NA=6.022×1023 atoms / molecules.
So number of atoms is given by:
(mass (grams)*N_A)/("Relative atomic mass", A_r)mass(grams)⋅NARelative atomic mass,Ar
In this case
((16.26times 10^-3g)/A_r)*(6.022 times 10^23)=1.66 times 10^20(16.26×10−3gAr)⋅(6.022×1023)=1.66×1020
Rearranging gives:
A_r= (16.26times 10^-3g)*(6.022 times 10^23)/(1.66 times 10^20)Ar=(16.26×10−3g)⋅6.022×10231.66×1020
so
A_r=58.99Ar=58.99