Many mental math strategies rely on students being able to “jump” tens. For example, 10 more than 28 is 38. The fancy math term for this skill is counting by 10s off the decade. In other words, not just counting 10, 20, 30, 40, etc., but also counting 17, 27, 37, 47, etc. Students should be able to fluently count off the decade from any number.
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Don’t, however, forget about the concrete–>representational–>abstract (CRA) sequence of instruction. While the goal is to be able to count by 10s off the decade, which is basically rote counting, we also want students to understand the concept. A great way to introduce counting off the decades is to build a number, for example 27, using base-10 blocks. Then continue to add rods as students count by 10s. So they see why counting by 10s off the decade works. Why the ones stay the same and only the tens change. Tie this in with patterns on the hundred chart.
A great way to practice counting by 10s off the decade is using one of my favorite routines from Number Sense Routines by Jessica Shumway. The routine is called Count Around the Circle, but it works perfectly when kids are in line, too, like when they’re lining up for recess, waiting in line for the restroom, etc. You start with a number, say 14, and proceed around the circle counting by tens. So the first child would say 24, then the next would say 34, etc. When you get around the circle, you can start back around counting backwards by tens! I like it better than choral counting, because each child knows they are going to participate. Of course, you want to have supports, like a hundred chart, for students who might struggle.
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