Commercial college textbook publishers do not provide free solution manuals or do not make these manuals accessible to students. Publishers releasing textbooks under the increasingly popular open textbooks publishing model often release free solution manuals for both students and instructors. The open textbooks hosted by the OpenStax College platform are systematically accompanied by their solution manuals. For K-12, NCERT is a company that provides problem solutions and solving guides.
The open textbooks publishing model has been created in order to undermine the commercial publishers who charge very high sums of money for college textbooks while imposing usage restrictions. Open textbooks are peer-reviewed textbooks created by faculty, sometimes for use in a specific course they teach. The books are openly licensed, so they can be downloaded and distributed freely. Solution manuals are made available to students based on the same principle.
The large platform OpenStax College provides access to solution manuals under the supplemental resources section on each textbook's page. On other platforms that host open textbooks, such as Collegeopentextbooks.org, the authors sometime post the free textbooks but offer the solutions manuals only upon direct request, albeit still free of charge. For K-12, NCERT provides chapter-by-chapter solutions for problems on its website.