20. The Egyptians, replacing the area of a circle with the area of an equal-sized square, took the side of the latter as $\frac{8}{9}$ of the diameter of the circle. Find the approximate value of $\pi$ from this.
20. Since $\left(\frac{8}{9} d\right)^{2}=\frac{\pi d^{2}}{4}$, then $\pi=3.16$.