Determine the density of the object and the radius of the pen.

Mass of object ($m$) = 2.5 kg
Volume of object ($V$) = 480 cm³

Convert mass from kilograms to grams for consistency with cm³:

$$m = 2.5 \text{ kg} \times 1000 \text{ g/kg} = 2500 \text{ g}$$

Density ($\rho$) is calculated using the formula $\rho = \frac{\text{Mass}}{\text{Volume}}$.

$$\rho = \frac{2500 \text{ g}}{480 \text{ cm}^3}$$ $$\rho = \frac{250}{48} \text{ g/cm}^3$$ $$\rho = \frac{125}{24} \text{ g/cm}^3$$ $$\rho \approx 5.2083 \text{ g/cm}^3$$

Rounding to three significant figures:

$$\rho \approx 5.21 \text{ g/cm}^3$$

The final answer is $\boxed{5.21 \text{ g/cm}^3}$.

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The string starts at 2.5 cm on the ruler.
The string ends at 15.0 cm on the ruler.

$$\text{Length of string} = 15.0 \text{ cm} - 2.5 \text{ cm} = 12.5 \text{ cm}$$

The string is wound 6 times around the pen. This means the total length of the string is 6 times the circumference of the pen.

$$6 \times \text{Circumference} = \text{Length of string}$$ $$6 \times C = 12.5 \text{ cm}$$ $$C = \frac{12.5 \text{ cm}}{6}$$ $$C \approx 2.0833 \text{ cm}$$

The formula for the circumference of a circle is $C = 2\pi r$, where $r$ is the radius.

$$2\pi r = C$$ $$r = \frac{C}{2\pi}$$ $$r = \frac{12.5 / 6}{2\pi} \text{ cm}$$ $$r = \frac{12.5}{12\pi} \text{ cm}$$

Using $\pi \approx 3.14159$:

$$r \approx \frac{12.5}{12 \times 3.14159} \text{ cm}$$ $$r \approx \frac{12.5}{37.69908} \text{ cm}$$ $$r \approx 0.3315 \text{ cm}$$

Rounding to three significant figures:

$$r \approx 0.332 \text{ cm}$$

The final answer is $\boxed{0.332 \text{ cm}}$.

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