Human resource development is central to national growth18. However, national assessments (EGRA) indicate a learning crisis in basic mathematics, specifically in relational ratios, due to traditional lecture-based methods. This study aims to 1) evaluate the effectiveness of PBL lesson plans against the 75/75 criteria, 2) compare academic achievement before and after implementation, and 3) study students' opinions regarding PBL. A quasi-experimental, single-group pre-test/post-test design was used. The target group consisted of 25 first-year primary education students at Dongkhamxang Teacher Training College. Instruments included 3 PBL lesson plans, E-Books, tests, and questionnaires. Results: The effectiveness E1/E2 was 77.57/78.83, exceeding the 75/75 threshold. Post-test scores ("X" ̅=15.72) were significantly higher than pre-test scores ("X" ̅=10.6) at the 0.01 level. The 6-step PBL method fosters "learning by doing," enabling students to link real-world problems to mathematics, resulting in highly positive student feedback (=2.76)