The three isotopes of neon are Ne-20 with atomic mass 19.992 amu and % abundance 90.48%, Ne-21 with atomic mass 20.994 amu and % abundance 0.27% and Ne-22 with atomic mass 21.991 amu and % abundance 9.25%.
The average atomic mass of Ne can be calculated as follows:
M=M(Ne-20)×%(Ne-20)+M(Ne-21)×%(Ne-21)+M(Ne-22)×%(Ne-22)
On putting the values,
M=(19.992 amu)(0.9048)+(20.994 amu)(0.0027)+(21.991 amu)(0.0925)=20.180 amu
Thus, average mass of neon is 20.180 amu