Thanksgiving Fact Family
Lesson 10 of 12
Objective: SWBAT solve related addition and subtraction story problems. SWBAT explain how the addition and subtraction facts are related.
Setting Up the Learning
We have been looking at how addition and subtraction facts are related with our dot flap cards. Today we are going to look back to some story problems and see if the addition fact can help us with the subtraction story.
Addition and subtraction are opposites. In addition we get more. In subtraction we take away. But they are related. When we figure out how they are related, it will help us solve problems in our head.
How can the addition fact help me with the subtraction fact?
I am making plans for Thanksgiving. I have a story problem I need your help solving!
I invited 7 people to my house. Then I invited 8 more people. How many people did I invite?
I’ll have students solve this either on the rug with a partner or quickly at their desks. I don’t want them spending too long on the problem!
- How many people did I invite? What number sentence matches this problem?
The 15 people I invited came to my house, but then 8 had to leave. How many people are still at my house?
- What is happening in this problem?
- How is this problem similar to the problem we just read and solved?
- Partner talk: How could we use the first problem to help us solve this one?
- What number sentence could I write for the second problem?
- Why does the subtraction sentence start with 15?
- How are these number sentences the same? How are they different?
- How did you use the first problem to help you solve the second one?
Student Solution and Share
I’ll give each student a number line to write on to model what happened in the problems. I am choosing the number line here because the number line is another way of representing the opposite relationship of addition and subtraction.
We have already used cubes, dot cards and pictures to represent in the unit. This is aligned to common core because students are expected to represent problems in a variety of ways.
- Where will I start on my number line for the first problem? Why? What does the 7 mean?
- How many jumps will I make? What do those jumps represent? What does the 15 mean?
- Where will we start on the number line? Why will we start at 15?
- How many jumps will we make? Which way will we go? Why?
- What number will we land on?
Students solve 4 related addition and subtraction fact story problems: Thanksgiving Fact Family.*
Students get different numbers based on ability level. Most students will receive numbers under 20. Students who are ready for an extension will receive numbers under 50.
See Group C strategy for a student solving a Group C problem. Notice how she uses tens to solve and notices the commutative property as she is solving!
*I left the numbers out of the word problems so teachers could choose numbers that are most appropriate for their students.
I planned to have students share out one of the fact families. I ended on a nonexample that showed writing an additional number sentence (see reflection for more information) that was incorrect to see if students would notice: Thanksgiving Fact Family_The Nonexample.