To find the density of the toy, we need to use the formula for density, which is:
\[ \text{Density} = \frac{\text{Mass}}{\text{Volume}} \]
We are given that the mass of the toy is 25 grams. Now, we need to calculate the volume of the rectangular pyramid.
The formula for the volume \( V \) of a rectangular pyramid is given by:
\[ V = \frac{1}{3} \times \text{Base Area} \times \text{Height} \]
The base area \( A \) of the pyramid can be calculated as:
\[ A = \text{Length} \times \text{Width} = 4 , \text{cm} \times 5 , \text{cm} = 20 , \text{cm}^2 \]
Now we can substitute the base area and height into the volume formula:
\[ V = \frac{1}{3} \times 20 , \text{cm}^2 \times 6 , \text{cm} = \frac{1}{3} \times 120 , \text{cm}^3 = 40 , \text{cm}^3 \]
Now we can calculate the density:
\[ \text{Density} = \frac{\text{Mass}}{\text{Volume}} = \frac{25 , \text{grams}}{40 , \text{cm}^3} = 0.625 , \text{g/cm}^3 \]
Thus, the density of the toy is \( \boxed{0.625 , \text{g/cm}^3} \).