The purposes of this research were: 1) to develop a problem-based learning activity package on linear equations of two variables with an effectiveness according to the 75/75 criteria, 2) to compare the learning achievement before and after learning with the problem-based learning activity package on linear equations of two variables, and 3) to compare the students’ problem solving skills before and after utilizing the problem-based learning activity package on linear equations of two variables. The population used in this research was a group of 180 Mathayomsuksa 1 and 3 students from the schools in Klung District, Weluwan Network Coordination Group in the second semester of the academic year 2020. The sample was a group of 30 Mathayomsuksa 1 and 3 students from Bantrokgnong School under Chanthaburi Primary Educational Service Area Office 2, selected by purposive sampling. The research instruments included: 1) a problem-based learning activity package on linear equations of two variables, 2) a test of learning achievement on linear equations of two variables and, 3) a test on mathematical problem-solving skills. The statistics used for data analysis included: mean, standard deviation (S.D.) and T-test.
