Sliding Down the Subtraction Number Line
Lesson 1 of 3
Objective: SWBAT deepen understanding of subtraction using a variety of ways to represent the quantities and process.
Sometimes I am like a giant kid, I must admit, and I am just pleased as punch about these sliding number line sandwich bags. We walk in from lunch, and instead of beginning the lesson with a typical lesson introduction, I show off my baggies! I ask my helper of the day to hold up a sliding zipper baggie, which she happily obliges, and I ask the kids, “How can we use that to do our subtraction?!”
A few kids propose some nice ideas, like “It can hold your counters,” but no one proposes anything to do with a number line! So, I show them the number line baggie, with classic kindergarten responses of “Oo”s and “ah” s.
“Let’s practice!” I suggest, and I call up all the kids who are sisters. Nine girls come up to the front of the group, and we talk briefly about the sisters’ siblings, just to be precise (MP.6) We slide the slider to 9 on the baggie, which is projected to our group on “the big screen”
Next, I ask all the little sisters to move over to a different area. There’s some minor confusion, as a few girls are both little sisters and big sisters, but I clarify to state that anyone who can call herself a big sister is a big sister—even if she’s also a little sister.
We count the little sisters as they walk away from the group, and a friend slides the slider back 4.
“What’s our answer?” I ask. Students yell, “5!” and we look at the 5 girls still standing, and the projected slider over the number 5 on “the big screen.” (MP.4) (It’s kind of a visual learner’s heaven, right there.)
For learners with different modalities, we sign and say, “9 [with a sign language 9] minus [gesturing a forearm horizontally to the right to be a minus sign] 4 [sign language 4] equals [2 vertical forearms for equal sign] 5 [sign language 5].” There’s still some visuals here, but we are verbally saying the equation and we are kinesthetically moving to form the equation.
My helper of the day—my partner—and I model how to take turns with the slider bag. I give a student a choice of 2 numbers, like “8 or 9?” I ask, quickly, and wait for student response.
After the student says “8,” my buddy writes 8 and slides our slider on the baggie to 8, which is projected on “the big screen” for everyone to see.
Next I ask a different student, “3 or 4?” After the student responds, I write the number in the second box of the equation and slide the slider back 3 from the 8, projecting it all on “the big screen.”
“What’s the answer?” I ask. My partner says, “5!” and I have her point to the 5 on the number line with her finger. We state together, “8 – 3 = 5.” (MP.2)
I have students work in pairs—carefully selecting the partners—partly because I think a peer will help keep some of the goofballs slightly more focused, and partly because I already used a few of my slider baggies from my box and I didn’t have 23 bags.
We continue this procedure for our guided practice, with students supplying numbers and sliding to use the number line.
For the other side of our recording sheet, I distribute spinners and number cubes, with the direction to spin the spinner to get the first number, and roll for the second number.
We take a moment for my helper of the day to come up and model this process with me, having her spin for the larger number, which she writes in the first box and shows on the number line baggie. Then I roll the number cube for the smaller number to take away, and I write that number in the box, then sliding it on the slider. Finally my partner points to the answer on the number line and writes it in the box.
We go over who does what in this process a couple of times, and then the partners switch. We “walk through” what needs to be done a few times, and then I set the students loose.
As with all practice, circulation and specific questions are critical. I don’t tell students they are wrong, but I definitely question when students need support. The most common misperception has to do with the number being subtracted and how it works with the number line. (MP.5) A student might have 9 – 4, for instance, and state the answer is 4. I ask guiding questions that lead the student to self-correct.
When I ask they about the trickiest part of math today, some students say it wasn’t tricky, (although between you and me, they have quickly forgotten some of their struggles!), and other students state that be just right—precise—with the slider and their numbers was trickiest. (My heart jumps with joy at the mention of “precise.”) (MP.6)
We talk at the end of the lesson about students’ favorite part of the lesson, and they enjoy sliding the slider, spinning the spinner, and even rolling the number cube. Yes, kindergartners are action-oriented people!