Saragih, Sahat, Syafari ., and Muliyono . “The Validity of Problem Based Learning Model To Improve Problem Solving Ability”. Advances in Social Sciences Research Journal 5, no. 2 (February 25, 2018). Accessed January 30, 2026. https://journals.scholarpublishing.org/index.php/ASSRJ/article/view/3755.