# Plus Ten Minus Ten

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## Objective

SWBAT add ten to a single digit number without counting up and subtract 10 from a double digit number.

#### Big Idea

To help students develop fluency skills with adding and subtracting numbers within 100. A variety of strategies need to be presented to encourage students to think mathematically.

## Warm Up

10 minutes

To encourage fluency with addition and subtraction and to connect new understandings to previous learning, I begin by asking students to write down the answers to several math problems:

• I have 10 eggs. I break 3. How many are left?
• There are 8 crayons in the box. I add 2. How many do I have now?
• I have 10 cookies. I give six to my friends. How many do I have left?
• I have 6 donuts. I buy six more. How many do I have now?
• There are 8 students in line. 8 more join the line. How many are in line now?

With each problem I ask students to give a thumbs up when they have the answer. While I encourage students to remember to rely on partners of ten or doubles to help them think about the answers, I also know that students will use other strategies that they are comfortable with. Students may use the number lines or number grids on their desks, or they may draw pictures, tally marks or solve the problems mentally.  After I have given all 5 problems, I ask for volunteers to explain how they got the answer.

I tell students that today we will look at what happens when we add 10 to a number and see if there are any patterns that can help us later on.

## Teaching the Lesson

30 minutes

I invite students to come to the rug to work at the interactive white board. Here I have a display of base 10 block tens and ones.  I put up 2 ones and then move a ten beside it.

What number do I have now? 12.

What is 2 + 10? 12.

I repeat this process with several other numbers such as 8 ones, move the 10 and now I have 18. Do students notice anything about adding ten to a number?

We discuss what students are seeing. The goal is for students to notice that the 1 from the tens moves into the tens place while the number in the "ones" place remains the same.

I repeat the process with subtraction. I start with 1 ten and 5 ones on the board. I remove the 10, how many are left? I repeat the process several times. I ask students what they notice?

If students are showing an understanding of this, I introduce adding 10 to a 2 digit number. I put up 14 and bring over a ten.

Now what number do I have?     24.      What happened?     The digit in the tens place went up one group of ten, and the number in the ones place stayed the same.

Again I repeat this process to help students visualize what is happening.

## Playing the Game/Independent Practice

25 minutes

I tell students that they will now be working in small groups. They will visit 2 of the 3 centers during the next 20 minutes. They will have a chance to play a game, and to work independently to solve some math problems using adding and subtracting 10.

The reason for having 3 centers, but students go only to 2 of them, is that it allows me to individualize. I can provide 2 different levels of independent practice. One group will work with the manipulatives to manually add 10 to or subtract ten from a variety of numbers, beginning with single digit numbers and moving to double digit numbers up to 20. This group will write their number sentences on a recording sheet.

The second group will have the manipulatives there to use if they choose, but the expectation will be that they will do most of the problems without them. This group will work with numbers to 50 when adding and subtracting ten.

I decide group membership based on my observation of previous understanding, and the understanding demonstrated during the teaching of the lesson.

Finally, each group will have a chance to play the game that practices adding and subtracting ten from single digit numbers. Students will be given blank BINGO boards. They will number the squares at random from 11 to 19. The caller ( a parent if possible so that I'm free to circulate) will hold up a number from 1 to 9. The student will look at the number, add 10 to it and cover that number on his/her game board. The caller will start out slowly, but then move more quickly through the numbers to encourage fluency with adding 10 to a number. The first student to win by getting 3 in a row will get a sticker on his/her paper. Play will continue until all students have 1 sticker.

For the second round the caller will hold up a card with the numbers 21 to 29 and students will be expected to subtract 10 from the number. Play will continue as above.

## Closing

5 minutes

After both groups have had a chance to play the game and complete the independent practice, students will return to their seats and take out their journals. They will complete the following statement in their journals: When I add 10 to a number, I.....  This will give some insight into their thinking about adding 10 to a number.