Note to the teacher:
This standard is incredibly important in first grade, because it naturally builds to second grade and beyond. In 2nd grade, adding and subtracting within 100 moves to the Operations and Algebraic thinking standards (CCSS.Math.Content.2.OA.A.1). Students have to apply what they are doing in first grade to more complex story problems, with unknowns in all positions. Additionally, students move to using place value concepts to add within 1000 (CCSS.Math.Content.2.NBT.B.7).
Read part of Roses are Pink, Your Feet Really Stink to get kids excited about solving problems about Valentine's day! This is a great time to incorporate high quality children's literature into math.
We have been solving story problems with big numbers! Today we are going to solve some Valentine's Day problems where we are giving away our Valentines. Great mathematicians have to think about how they add AND subtract big numbers. This helps them get ready for the big numbers they will use in second grade.
Your thinking job today is: What strategies can I use to solve subtraction story problems with big numbers?
The purpose of the opening discussion for this lesson is for students to start thinking about how to efficiently represent numbers so they can solve subtraction story problems.
Don't be surprised if students who represent with base 10 using addition use a less sophisticated strategy for subtraction! Anytime you change the problem type, students may go back to using individual cubes. This opening will help them plan out how they are going to solve.
Present Problem: I made 40 Valentines for my friends. I gave 30 of the Valentines out. How many Valentines do I still have?
This focus on tools is aligned to the Common Core MP5, "Use appropriate tools strategically". Students think about what tool makes the most sense for solving this problem. I make sure to have multiple tools available so students can choose one that makes the most sense for them. This also gives them practice in attempting one tool and then abandoning it if it isn't working! Little mathematicians need practice determining the most efficient tool!
Tools I have available: Number lines, hundreds charts, individual unifix cubes, base ten blocks
Student Work Time:
Students work for 7-10 minutes on this problem independently. Students represent what they did in a picture. They also write words to explain their thinking, aligned to the CCSS emphasis on writing across the curriculum.
I'll choose 2 strategies to share quickly with students. I'll choose a student who used individual cubes or counted out groups of 10 and another student who used base 10 blocks. The reasoning here is that students who are focusing on the individual cubes need a push to help them try base 10. The discussion we have is aligned to MP7, Look for and make use of structure. Students look for patterns and structure in the base 10 system. Showing both strategies will help those students see the connection between the two strategies!
Strategy 1: Using individual cubes
Strategy 2: Using Base 10 Blocks
After we share 2 strategies, I'll have students partner share.
Partner talk: Which strategy did you use? What strategy will you use at your desk?
Students work on Valentine problems at their desk. They each get 3 problems. However, I differentiate this work based on number.
Group A: Intervention
These students get numbers under 40, always on the decade.
Push: Push these students to try grouping their individual cubes into groups of 10. They are probably not ready to use base ten blocks yet. They need to build the towers!
Group B: Right on Track
Students get numbers to 90, always on the decade. These students are probably using base 10.
Push: Push these students to think about how they can count to make their strategy faster. For example: 60-20. When you take away 10, how many are left? What about when you take away another 10? (Pushing counting backwards by 10!)
Group C: Extension
Students get numbers to 100, but one number is not on the decade. This is an extension of this standard. Most of these students were probably using known facts to help them figure out 40-30 and it wasn't a challenge. Push them by giving them numbers like 73 - 40.
See attached pdf for independent practice. I left blanks for the numbers so you can write in the numbers you want each group to use!
Give each student a heart outline. Have them show how they solved one of their problems on the heart. Then they can send the Valentine to their kindergarten teacher and show them how much they have learned!
See attached pdf for a Valentine template! I'll have kids cut the heart out and color the other side of it!