Phuphasit, K., and P. Prasertsang. “Developing Problem Solving Skills in Mathematics of Grade 4 Student by Using Constructivist Theory”. Journal of Roi Et Rajabhat University, Vol. 14, no. 3, Dec. 2020, pp. 141-8, https://so03.tci-thaijo.org/index.php/reru/article/view/239140.