20. Given that the length of the major axis of an ellipse is 4, and the focal distance $\left|F_{1} F_{2}\right|=2$, the sum of the lengths of two perpendicular chords passing through the focus $F_{1}$ is $\frac{48}{7}$. Then the product of the lengths of these two chords is $\qquad$
20. $\frac{576}{49}$