1. **Stating the problem:** We need to find the length of side $BD$ in a triangle or geometric figure where $BD=15$ is given. 2. **Formula and rules:** Since the problem references a previous question but only changes $BD$ to 15, we assume the problem involves calculating lengths or using the Pythagorean theorem or triangle properties. 3. **Intermediate work:** Without the original figure or other side lengths, we focus on the given $BD=15$ as a fixed length. 4. **Explanation:** If the previous problem involved calculating $BD$ and now $BD$ is given as 15, then this is the length of segment $BD$. Any further calculations would depend on other given sides or angles. 5. **Final answer:** The length of $BD$ is $15$ units.