G5.3 In Figure 6, a square-based pyramid is cut into two shapes by a cut running parallel to the base and made $\frac{2}{3}$ of the way up. Let $1: c$ be the ratio of the volume of the small pyramid to that of the truncated base, find the value of $c$.
The two pyramids are similar. $\frac{\text { volume of the small pyramid }}{\text { volume of the big pyramid }}=\left(\frac{x}{3 x}\right)^{3}=\frac{1}{27}$ $c=27-1=26$