Adding to 100 by tens and ones without regrouping

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Students learn to add to 100 by adding the second addend in tens and ones to the first addend.

2.NBT.B.5

2.OA.A.1

Discuss with students that you can use adding by tens and ones when the numbers you are adding are too big to do at once in your head. An example would be if you take a train and there are 41 people on the train, and 26 more get on board.

Ask students to count by turns. Start using the given number and time how long it takes them to reach 100. Repeat this with a different starting number, and time them again. Ask students if they think they were faster the second time or the first. Next show students a number line with a few steps shown. Ask students what the total is.

First explain how adding by tens and ones works. You split the second addend into tens and ones, and you add them to the first addend separately. Then remind students that tens are a group of 10 and ones are the numbers from 1 to 9. Explain that you can add by tens and ones using the number line. The first addend goes at the start of the number line. Discuss the steps that you can take on the number line to represent the second addend. Show students a number line with different steps shown on the number line. Ask them if they can determine the total. Next practice another problem where the steps are first in tens and then ones. Say that you can also add by tens and ones without using a number line. To do so, you split the number into tens and ones. First you add the tens to the first addend. When you have this total, you then add the ones to the intermediate number. Ask students if they can determine the totals of a few given addition problems. Ask students if they can explain how to add by tens and ones. Erase the grey boxes to check their answers.

Check that students understand adding to 100 by tens and ones by asking the following questions:

- What does it mean to add using tens and ones?

- What is the first step you take when you do this?

- How do you solve the problems 34+13 and 62+37?

Students first practice adding with tens and ones with support of a number line. Then they calculate without the visual support.

Discuss with students that you can use adding by tens and ones to make it easier to calculate larger addition problems. Ask students to solve a few exercises using the number line. They must write their own number lines and then compare them. By doing so, they can see that there are different ways to step to the answer. Finally students write the addition problems and solve the answers. Ask them to explain their thinking.

Students who have difficulty adding with tens and ones can make use of the number line. Have them practice splitting the second addend into tens and ones, and then making those steps on the number line. Emphasize that they first jump in tens, and then in ones on the number line.

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