To find the volume of a pyramid, you can use the formula:
\[ \text{Volume} = \frac{1}{3} \times \text{Base Area} \times \text{Height} \]
Given that Cecilia's pyramid has a rectangular base, we can calculate the base area by multiplying the length and width of the base:
\[ \text{Base Area} = \text{Length} \times \text{Width} = 8 , \text{inches} \times 12 , \text{inches} = 96 , \text{square inches} \]
Next, we can plug the base area and height into the volume formula. The height of the pyramid is given as 20 inches:
\[ \text{Volume} = \frac{1}{3} \times 96 , \text{in}^2 \times 20 , \text{in} \]
Now, let's do the calculation:
\[ \text{Volume} = \frac{1}{3} \times 96 \times 20 = \frac{1920}{3} = 640 , \text{cubic inches} \]
So, the volume of Cecilia's model pyramid is
\[ 640 , \text{in}^3 \]