The weighted average atomic mass is calculated by summing the products of the atomic mass of each isotope and its fractional abundance.

$\text{Average Atomic Mass} = \sum (\text{Mass}_i \times \text{Fractional Abundance}_i)$

1. Convert Percentages to Decimals: $^{200}$X: $90\% = 0.90$ $^{199}$X: $8.0\% = 0.08$

• $^{202}$X: $2.0\% = 0.02$

2. Calculate Weighted Contributions: $200 \times 0.90 = 180.0$ $199 \times 0.08 = 15.92$

• $202 \times 0.02 = 4.04$

3. Sum the Contributions: $180.0 + 15.92 + 4.04 = 199.96 \text{ amu}$

4. Round to Nearest Integer: The value $199.96$ is closest to $200$ amu.