Answer
$10.\,u$
Work Step by Step
75 g corresponds to 7.5 moles of atoms
$\implies 75g\,corresponds\,to\,7.5\times6.02\times10^{23}\,atoms$
$=4.515\times10^{24}\,atoms$
Average mass of the atoms=$\frac{75\,g}{4.515\times10^{24}}=1.66\times10^{-23}g$
In atomic mass units,
Average mass of the atoms=$1.66\times10^{-23}g\times\frac{1\,u}{1.66\times10^{-24}g}=10.\,u$
