Counting on to add and subtract - math grade 2 - lesson 12

Note to the Teacher: There are three parts to today's Learn section (identifying the bigger number, counting on from the bigger number in addition, and counting on from the smaller number in subtraction). You may find that some students will need more time with one part, so slow the pace if needed or extend the lesson to the next day.

TEACHER DO: Write two numbers on the board (for example, 7 and 12).

TEACHER SAY: Which of these two numbers is bigger?

STUDENTS DO: Call out answer.

Note to the Teacher: If students do not understand what "bigger number" means, you can explain that it means the one with a larger quantity. Would students rather have 7 pieces of candy or 12, for example.

TEACHER SAY: That is right. 12 is bigger than 7.

TEACHER DO: Repeat this a few more times to ensure all students are able to quickly identify the bigger number.

2.TEACHER SAY: Great work. Now we are going to practice adding these numbers by using a mental math strategy called Counting On. If we see 7 + 12, it is easier for our brains to start with the bigger number—12— and then count on the smaller number—7. You can use your fingers or look at a number line to help you. Let's try this together.

TEACHER DO: Write 7 + 12 = on the board. Start at 12, and then hold up 7 fingers and count on 1 for each finger.

TEACHER SAY: When I added, I actually started with 12 because it is the bigger number. Then I counted on 7 more to get 19. Let's try a few together. Remember to use the Counting On strategy.

TEACHER DO: Write a few more addition problems on the board (addends up to 20). Walk the class through each one by asking them to identify the bigger number, then counting on the smaller number to find the answer.

STUDENTS DO: Use the Counting On strategy to solve addition problems with the teacher. Call out together to answer when prompted.

3.TEACHER SAY: You are doing a great job. We can also use the Counting On strategy to solve subtraction problems, but it is a bit different. When we ADD two numbers together, it is easier to start with the bigger number. When we SUBTRACT two numbers, it is easier to start with the smaller number and count on to the bigger number to find the difference. Let me show you.

TEACHER DO: Write 12 – 7 = _____ on the board.

TEACHER SAY: Which is the smaller number?

STUDENTS DO: Call out the answer.

TEACHER SAY: That is right. 7 is smaller, so now I will count on from 7 until I get to 12. Watch.

TEACHER DO: As you count from 7 to 12, hold up one finger for each number you say.

TEACHER SAY: Th e tricky part here is to remember to start putting your fi ngers up AFTER

you say the smaller number in the problem. I did not put a fi nger up until I said 8. Watch me again.

TEACHER DO: Model the solution to 12 – 7 again. Emphasize that you do not start counting on until after you say 7.

TEACHER SAY: Let's try a few problems like this together. Remember to use the Counting On strategy, but this time start with the smaller number and count on to the bigger number.

TEACHER DO: Write a few more subtraction problems on the board (all numbers up to 20). Walk the class through each one by asking them to identify the smaller number and then count on to fi nd the diff erence.

STUDENTS DO: Use the Counting On strategy to solve subtraction problems with the teacher. Call out together to answer when prompted.

4.TEACHER SAY: You are all doing a fantastic job using the Counting On strategy. Now please take out your student book and turn to the page Lesson 12: Apply. Solve the addition and subtraction problems on your own using the Counting On strategy. Pay attention to the signs because some of these problems are addition and some are subtraction.

STUDENTS DO: Work independently to apply the Counting On strategy to solve problems in the student book.

TEACHER DO: Walk around the room and take note of students who are struggling with this concept. At the end of the Learn segment, have students compare their answers with those of their Shoulder Partners and talk about how they solved the problems.