This paper probes, through a Lesson Study, how discovery learning and contextualized problems can be employed in teaching the mean of grouped data. The research lesson was implemented Grade 7 students of a national high school in Calamba City, Laguna, Philippines. Through thematic analysis, three issues emerged from conducting the Lesson Study: (1) making meaning about the data in context fosters students’ mathematical understanding; (2) translating 21st century learning in the classroom with technical topics like the mean of group data can be done by shifting the focal point of the lesson on its importance and applications in life; and (3) identifying the critical points of a lesson is an indispensable process that teachers need to undergo in designing learning activities that promote 21st century learning and relevant to the demands of the Fourth Industrial Revolution. This study demonstrated how Lesson Study is an effective professional development intervention that allows teachers to view the learning process in a wider perspective, strengthen their skills in designing efficacious lessons, and bring forth change in their teaching practices via reflective stances on lessons.