Pisapak, Jittima, and Siriporn Thipkong. 2009. “The Development of Mathematics Ability in Solving Problems for Prathomsuksa 2 Students at Kasetsart University Laboratory School by Using Polya’s Problem Solving Process”. Kasetsart Journal of Social Sciences 30 (1):1-9. https://so04.tci-thaijo.org/index.php/kjss/article/view/246479.