Neon has three stable isotopes. Neon-20 has an atomic mass of 19.992 u and makes up 90.48 % of the…

Step 1. The contributions. Neon-20: $0.9048 \times 19.992\ \text{u} = 18.089\ \text{u}$. Neon-21: $0.0027 \times 20.994\ \text{u} = 0.057\ \text{u}$. Neon-22: $0.0925 \times 21.991\ \text{u} = 2.034\ \text{u}$.

Step 2. The sum. $18.089\ \text{u} + 0.057\ \text{u} + 2.034\ \text{u} = 20.18\ \text{u}$.

Step 3. The count. $\dfrac{9.25}{100} \times 10\,000 = 925$ atoms.