The Moon's density is $3.3 \mathrm{kg} / \mathrm{L}$. Suppose that the Moon is made of just two substances: silicates with a density of $3.0 \mathrm{kg} / \mathrm{L}$ and iron with a density of $7.9 \mathrm{kg} / \mathrm{L}$
a. What fraction of its mass must be iron?
b. What fraction of its volume must be iron?