Solving Subtraction Number Stories- Under the Sea

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Objective

Students will be able to solve subtraction word problems using objects.

Big Idea

When introducing subtraction, students can become easily overwhelmed with numbers and symbols. In this lesson, students will learn using cubes to solve subtraction word problems related to under the sea.

Problem of the Day

5 minutes

I start each math lesson with a Problem of the Day.  I use the procedures outlined here on Problem of the Day Procedures.

Today's Problem of the Day:

Jenny has 6 pieces of cake.  She gave 2 of them to her friends.  How much cake does Jenny have left?

I set this problem up with some structures to help the students organize their thinking.  On the Notebook file, there is also a picture of a piece of cake set to Infinite Cloner.  This way the student can use it represent the problem.  If you do not have a SMART Board, you can use the PDF and manipulatives, pictures, or students' drawings.

Since we do this whole group, I have one students come up and work on this problem.  I remind the student to check their work when they are finished and have the class tell if they agree or disagree by showing a thumbs up or thumbs down.  Students also have the opportunity to share why they agree or disagree.

Presentation of Lesson

25 minutes

I start this lesson by handing out a Ocean Mat to each student and having them take it back to their seat.  This mat is available to in color and black and white.  I use the same mats for introducing addition and subtraction, so I print them in color, put them in plastic sleeves and collect them at the end of the lesson addition unit to save for this lesson.  I put one copy on the document camera and project it on the SMART board so that I can model for all of the students to see.

I also give each student a train of 10 counting cubes.  I have the cubes connected into trains ahead of time so that they are counted out and easy for the students to carry back to their seats.  I tell students that when they get back to their seats they need to take the cubes apart, lay them beside their paper and then put their hands on their head.

Today I am going to tell you some stories.  Each story ends with a question.  We are going to use the cubes to answer the question at the end of each story.   Stories like this that include a math problem are called word problems.  While I tell you the story, use the cubes to show what is happening in the story.  These are different than the word problems we have been working on because this time we are going to be taking something away in each problem.  See example here.

I say the following word problems aloud.  As I say them, I model on my mat using the cubes and have the students model the problem on their mats.  I ask for a student to orally give me the answer by raising a quiet hand.  We do not write down anything during today's lesson.  I want the students to focus on listening to the problem and using the cubes to find the answer.  The process is just as important as whether the students get the correct answer.  The students need to use several of the mathematical practices to do this.  They need to make sense of the problem and engage in finding the answer (MP1).  They need to use the cubes to model the problem (MP4), and they need to use the cubes as math tools (MP5).  At this level, cubes and other manipulatives are math tools that students need to learn to use appropriately to solve problems.

 

Three fish were swimming in the ocean.  Two fish swam away.  How many fish are left?

There are five starfish sitting on the sand.  One gets picked up by a diver.  How many starfish are still sitting on the sand?

Seven people go diving in the ocean to look for treasure.  Five go back to the boat.  How many divers are still looking for treasure?

Eight mermaids are swimming in the ocean.  Two go up to the surface.  How many mermaids are left?

Ten little fish are swimming.  Three fish get gobbled up by a big fish.  How many fish there now?

I let two students come up with ideas for problems for the class to solve.

 

When we are finished, I ask each student to put their cubes back together into a train.  They need to count and make sure that they have 10.  They then need to make a pile of papers and cube trains in the middle of their table.  I select one student to collect the papers and another student to collect the cube trains.  When students are finished I call one representative from each table to come back and get the center for their table.  All students start their center at the same time today.

Practice

20 minutes

The centers for this week are:

 

I quickly circulate to make sure students are engaged and do not have any questions about how to complete the centers.  Because of the trouble during our addition unit, I decided not to introduce any subtraction centers this early in the subtraction unit. 

I pull 3 groups (5-10 minutes depending on need).  We do an activity similar to the whole group activity.  I have the students use bears and fruit counters instead of cubes to show that we can use a variety of manipulatives to represent the information in a number story.

Prior to clean up, I check in with the other tables to see how the centers are going.  My students have been struggling with getting cleaned up quickly and quietly after centers.  Lately I have been using counting down from 20 slowly instead of a clean up song.  Counting backwards is as critical as counting up. Students need to be able to know the number that comes before, as well as after, any given number (w/i 10, w/i 20, etc.). Counting back is a critical strategy for subtraction.

The students like to count backwards with me as they clean up and I can lengthen or reduce the clean up time based on how students are doing and how much time we have.

 

Closing

5 minutes

To close, I put the space mat on the document camera a project it on the SMART Board and have that student show a subtraction word problem and explain their work.  I mention positive things noticed during centers as well as something that needs to be better next time. 

I review what we did during our whole group lesson.  "Today we learned about solving problems that come from a story.  These are called word problems.  Tomorrow we will be continuing to work on word problems, but we are going to start writing the answer to the problems."