15. Let $F_1, F_2$ be the two foci of the ellipse $C: \frac{x^2}{16} + \frac{y^2}{4} = 1$. Let $P, Q$ be two points on $C$ that are symmetric about the origin, and $|PQ| = |F_1F_2|$. Then the area of quadrilateral $PF_1QF_2$ is $\_\_\_\_$.\\
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