Seen from the two sides of the Earth's orbit, a star shows a parallax of 0.25 seconds of arc (the…

Step 1. Angle. $\dfrac{0.25^\circ}{3600} = (6.94 \times 10^{-5})^\circ$ degree, and $\times \dfrac{\pi}{180}$ gives $\theta = 1.212 \times 10^{-6}\ \text{rad}$.

Step 2. Metres. $D = \dfrac{1.496 \times 10^{11}\ \text{m}}{1.212 \times 10^{-6}\ \text{rad}} = 1.23 \times 10^{17}\ \text{m}$.

Step 3. Parsecs. $D = \dfrac{1}{0.25} = 4.0$ parsec.

Step 4. Light years. $\dfrac{1.234 \times 10^{17}\ \text{m}}{9.46 \times 10^{15}\ \text{m}} = 13.0$ light years.