Ideas on Subtraction
Lesson 3 of 13
Objective: SWBAT use a strategy to solve subtraction problems. SWBAT model the action of subtraction with counters or pictures. SWBAT record their subtraction strategy.
Advanced preparation: You will need to print out both sets of number cards from the resource section. Cut out the 1-30 cards and put them into one envelope and do the same with the 31-60 and put them in their own envelope.
I start this part of the lesson by asking the kids to sit in front of the classroom number line.
"Today we are going to change up our Start At/Stop At routine. We are going to add the numbers 31-60." I will pull one card out of the 1-30 envelope and we will use that as our start at number. I will then pull out a card from our 31-60 envelope and use that as our Stop At number. We will then coutn as a class from our starting num,her to our ending number."
I will ask a student to point to each number as we count as a whole group. I will continue this process as time allows. I will also mix in counting backwards by starting at the higher number and counting to the lower number. The Core Standards expect 1st graders to be able to "count to 120, starting at any number," by the end of 1st grade. This routine is the process in which I can assure that the students are continuously working toward that standard (CCSS.Math.Content.1.NBT.A.1).
Read the following word problem to the students using the same routine that was used in the previous lesson (once you go to this link you will want to look in the section titled Solving Subtraction Word Problems):
"Tom picked 12 oranges. He gave 6 of them to Kim. How many apples did Tom have left?"
Then ask each student to retell the story to a neighbor. After that have two or three students retell the story in their own words to the group.
Then explain: "Everyone will start Center Time by solving this problem. Once you are done you can choose one of the other activities from the Center Time choices."
"Please make sure to record how you solved the problem and to write an equation. Remember I want to be able to look at your paper and know exactly what you are thinking."
I then let the kids find a spot in the room (by themselves) to work in this task. A copy of the story problem is int he resource section.
The CCSS Math Practices expect students to "make sense of problems and persevere in solving them. Mathematically proficient students start by explaining to themselves the meaning of a problem and looking for entry points to its solution (CCSS.Math.Practice.MP1)." The above problem is an example of giving students an opportunity to use their skills to develop this practice.
The students will work on the word problem activity first. Then they can choose from the other two choices as they finish. Either of the other two choices can be played as a partner game or individually.
Center Time Choices:
1. How Many Oranges word problem: Students will work individually on the word problem. As students are solving this you should circulate to see how students are doing with making sense of the action in the problem and how they are recording their thinking. You will want to identify the strategies that are being used and who efficient each student is with their choice.
2. Subtraction Bingo: To play this game the students will need a copy of the Subtraction Bingo recording sheet. Each student(s) will also need one 1-6 dot die and a 7-12 numbered die.
3. Dice Subtraction: To play this game you will need the Dice Subtraction recording sheet and the same two dice that were used in the Subtraction Bingo activity. The student(s) roll the two dice and subtract the dot die from the numbered die. Then the student(s) write the subtraction fact on the recording sheet. The game is over when one column is filled. There is a video of a student playing this game and an example of a filled out sheet in the resource section.
Activities 2 & 3 both have students working on subtraction fluency. The Core Standards expect first graders to "relate counting to subtraction and subtract within 20, demonstrating fluency for subtraction within 10 (CCSS.Math.Content.1.OA.C.5 & CCSS.Math.Content.1.OA.C.6)." These two activities allow for the opportunity to work on these skills and take steps toward fluency.
Lesson Wrap Up
As students were working on the How Many Oranges? problem, I was looking for an example of each of the following strategies:
1. Counting all, removing a group, and then counting what was left.
2. Counting back or down on a number line.
3. Using a number relationship they already know.
I have included an example of each in the section resource. I ended choosing a student who used a number line and one counted back because that is what I wanted students thinking about as the discussion ended. As each student presented, I wanted to make sure the standard notation was emphasized with each strategy. It is important to relate the students "action" with the notation that represents their thinking. First graders must be able to represent their thinking with mathematics (CCSS.Math.Practice.MP4).
To end the lesson, I asked the students to partner up and take turns rolling a 20 sided die. The roller would have to say the neighbor numbers of the number rolled and the partner would have to say the number (i.e. I rolled a 16. I would say 15 & 17 and my partner would have to say 16).
This is part of my continued practice of number and operation skills that our district tests the kids on at the end of the year. I find it an easy way to supplement the end of the math lesson.