Number units can be harder to do in a combined setting. People deal with this challenge in a variety of ways. Having many activities that students can work on independently frees up the teacher to work with groups of children, catering more to the needs of individual students and small groups. I wanted my kids to be tackling. These units are written to follow each other and extend the ideas of the earlier unit.
Investigations offers a large amount of both independent and group work.
So I found that, for example, when I had students working on multiplication word problems I could use many of them right from the book but then adjusted some. I found the same to be true for many of the games that provide important practice in computation. For example, The 3rd graders are expected to become fluent in the multiplication combinations up to 6x6 while the 4th graders are expected to learn up to 12x12. During Choice Times that are written into the curriculum, I had time to work with a specific group of students who needed support or extra challenge. I like that some students can move on to more challenging work while others will have another year to continue working on and developing some of their ideas.