20.10 amu
Neon consists of 3 isotopes with masses 19.92, 20.99, and 21.99 amu. The relative abundance of these 3 isotopes is 90.92, 0.25, and 8.83 percent, respectively.The average atomic mass of neon is the weighted average of the mass of the isotopes and equal to 19.92*0.9092 + 20.99*0.0025 + 21.99*0.0883 = 20.10
