Answer
$$3.83 \space g$$
Work Step by Step
As we have calculated in the last exercise (2.98a), the mass of the necklace is equal to 42.5 g.
$$100 \% \space necklace = 9.0 \% \space copper$$
$$42.5 \space g(necklace) \times \frac{9.0 \% \space copper}{100 \% \space necklace} = 3.83 \space g(copper)$$
