Choose an Operation

To start this lesson, I place 14 counters in a cup and gently shake and spill all the counters from the cup.  Have a student count the red and then the yellow counters.  I ask:

• How many red counters are there?
• How many yellow counters are there?

I have the student write an addition sentence on the board/chart paper.  I repeat the activity changing the number of counters to another number through 20.  This will get the students thinking about addition and subtraction sentences.

I then write the following addition and subtractions sentences on the board/chart paper, but leave the plus sign and minus sign blank: (also available in resources Choose an operation)

•   7 ____ 8 = 15                                  14 _____ 7 = 7
• 17 _____ 8 = 9                                    8 _____ 5 = 3
• 12 _____ 9 = 3                                    6 _____ 9 = 15

I ask student volunteers to write the plus or minus sign to make each sentence true.  Then I ask the students:

• How did you know whether to add or subtract?  (In order to make both sides of the equal sign equal in the first problem, I know I have to add 7 and 8.)

I explain to children that in today’s lesson they will choose the appropriate operation to solve story problems.

This lesson allows children to explore and demonstrate their understanding of addition and subtraction. The word problems ask children not only to choose an operation, but also to explain their solution (MP3). The lesson encourages children to choose a way of explaining that works for them. Children may use objects, draw pictures, or write to explain their work. For each problem, I give several children the opportunity to justify their reasoning.  Children develop reasoning and communication skills by presenting ideas and by listening critically to others. Children might ask themselves, Does what I am hearing and seeing make sense to me? Is it correct? Is there another way to solve the problem?  Encourage children to ask questions of each other: How did you know how many to subtract? How did you know how many to add? Why did you solve it that way?

I start this lesson by writing the following problem on the board/chart paper: (also available in the resources Choose an operation)

Jane has 13 computer games.  There are 8 adventure games.  The rest are math games.  How many math games does Jane have?

I guide the students to think about the correct operation using the following questioning:

• What information do you have?  (Jane has 13 computer games, 8 are adventure games, the rest are math games)
• What do you need to find? (The number of math games Jane has)
• Will you add or subtract to solve the problem?  How do you know?  (I will subtract because the number of math games is the part that is left after subtracting the number of adventure games.)
• How can you use counters to solve the problem? (I count out 16 counters, and then I move 8 of the counters away.)

After solving the problem, students draw and display a picture to represent how they solved the problem.  By having students draw a picture, it will help them to move from the concrete to the abstract aspect of solving word problems.

I then write the following problem on the board/chart paper:

Mary sees 8 squirrels.  Jack sees 9 more squirrels than Mary.  How many squirrels does Jack see?

I tell the students that some problems can be solved using either addition or subtraction.  I ask these questions as you work through the story problem together:

• How do you decide to add or subtract to solve the problem? (Mary sees 8 squirrels.  Jack sees 9 more than Mary, so I have to add 8 and 9 to find how many Jack sees.)
• Could you solve this problem with subtraction? Explain. (No, I only know the two parts, I don’t know the whole.)
• What strategy would you use to solve this problem? (I would count on from 8; I would count out 8 counters and add 9 more.)

I have students work with shoulder partners to solve this problem.

For the independent practice portion of this lesson, I have students answer the word problems using Choose and operation_worksheet.  Part of the assignment will be for the students to determine if they need to add or subtract in order to solve the problem.

In this video, students are using the strategy of counting on to answer an addition problem:

For struggling students, write the following problem on the board/chart paper:

• Jane’s team has 13 children.  Some of the children go home.  Now there are 7 children left on the team.  How many children went home?

Have the students count out 13 counters, and then remove 7 of them.  Ask:

• How many counters do you still have? (6)
• Would you add or subtract to solve the problem? Why? (I can count on from 7 until I get to 13.  That shows how many children went home.  I would subtract.  I can subtract the number left from the starting number to find the number of children that went home.)

Repeat with similar stories.

In this picture, a student is using counters to help with modeling the math problems.