Mathematical reasoning and problem-solving is the heart of mathematics (Hasanah et. al, 2019). The great part about co-teaching is that it allows more opportunities for differentiation, discovery and problem-solving due to having two teachers that can offer students' opportunities. Challenges Combining Co-Teaching and Research-based Methods Unfortunately, according to Indriati (2018), there exists three challenges for blending co-teaching with research-based methods for teaching mathematics. The first challenge exists in "identifying research-based practices for specific mathematics content areas, particularly for algebra, geometry and data and probability". The second challenge is that "some instructional practices may not be appropriate for all individuals and their level of content knowledge". The third challenge is that "some research-based practices will not work well in a co-taught general education setting" (p. 4). Therefore,...