Developers reuse programming-related knowledge on Q&A sites that functionally matches the programming problems they encounter in their development. Despite extensive research on Q&A sites, being a high-level and important type of development-related knowledge, architectural solutions and their reuse are rarely explored. To fill this gap, we conducted a mixed-methods study that includes a mining study and a survey study. For the mining study, we mined 984 commits and issues from 893 OSS projects on GitHub that explicitly referenced architectural solutions from SO and SWESE. For the survey study, we identified practitioners involved in the reuse of these architectural solutions and surveyed 227 of them to further understand how practitioners reuse architectural solutions from Q&A sites in their OSS development. Our findings: (1) OSS practitioners use architectural solutions from Q&A sites to solve a large variety of architectural problems, wherein Component design issue, Architectural anti-pattern, and Security issue are dominant; (2) Seven categories of architectural solutions from Q&A sites have been reused to solve those problems, among which Architectural refactoring, Use of frameworks, and Architectural tactic are the three most reused architectural solutions; (3) OSS developers often rely on ad hoc ways (e.g., informal, improvised, or unstructured approaches) to incorporate architectural solutions from SO, drawing on personal experience and intuition rather than standardized or systematic practices; (4) Using architectural solutions from SO comes with a variety of challenges, e.g., OSS practitioners complain that they need to spend significant time to adapt such architectural solutions to address design concerns raised in their OSS development, and it is challenging to use architectural solutions that are not tailored to the design context of their OSS projects.