Kummod, Arisa, and Sitthipon Art-in. 2019. “The Development of Mathematics Problem Solving Ability for Grade 12 Students Through Open Approach With Polya’s Problem Solving Process”. Journal of Education Khon Kaen University (Graduate Studies Research) 13 (2):125-35. https://so02.tci-thaijo.org/index.php/EDGKKUJ/article/view/216861.