In first grade, students are expect to add and subtract within 100 (1.NBT.C4, 1.NBT.C6). This lesson is very much an extension of these standards. Students apply their understanding of different kinds of word problems, as well as their understanding of place value, to solve these problems. This lesson can be used as an extension in first grade, or as a lesson to align with 2.OA.1 in 2nd grade!
Read Aloud & Hook:
How can I use what I know about 10 to help me solve for the missing part?
Present Problem: There are 64 people on the bus. When the bus stops, a group of friends get off the bus. Now there are 35 people on the bus. How many people got off the bus?
Student Work Time: 8-10 minutes
As students work, I am circulating, looking for student strategies to share. You can see one student's strategy here: Student Work Example. It shows how this student used base 10, then broke the ten. Most students will directly model, which means they will act out the problem using a concrete model, if this is one of their first times seeing a problem like this. The student work example shows how students can represent their thinking in a picture!
I'll choose 2-3 students to share their strategies with the class. I am anticipating that most students will directly model this problem, meaning they will take out the 64, and take away until they get to 35. However, I also want to see if students start with 64 and just take away 35. This would show that they have an understanding of a property of subtraction that I would want to explore as a class.
If this happens, I'll ask the following questions to students:
After strategy share, I'll have students partner talk about how the strategies were the same and different.
Students solve problems on their own using the packet: Bus Base 10 Independent Practice. They are either applying strategies they heard from the debrief or practicing a strategy they already used today. However, during this time I'll differentiate the number set based on what I saw in the student share problem.
Group A: Intervention
I'll have these students work with numbers under 50, on the decade. These students probably struggled when they were solving earlier, particularly getting stuck on trying to handle the number set.
Group B: Right on Track
Students will get numbers under 75. Students may have to compose/decompose a ten like they had to in the student share. For the missing addend problem, I'll use 54 + ____ = 70. For the missing subtrahend, I'll use 63 - ___ = 39
Group C: Extension
Students get numbers under 100. Students have to compose/decompose a ten like they had to in the student share. Examples: 76 + ____ = 100; 93 - ___ = 64
Students complete the Base 10 Assessment to show their understanding of base 10. It includes problems to assess how they solve 10 more and 10 less problems, as well as a story problem where they show how they subtract within 100.