Addition to 100 by tens and ones using addends >10

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Students learn to add to 100 by tens and ones, which means they split the second addend into tens and ones and add those separately to the first addend.

2.NBT.B.5

Discuss with students that they can use adding by tens and ones to solve addition problems that are too difficulty to solve in your head in one go. An example is if if 38 cars are waiting for the light to turn green and 18 more stop at the light, you can use adding by tens and ones to determine that 56 cars are waiting for the light to turn green.

Ask students to determine whether the given statements about decomposed numbers are true or false. They stand for false, and sit for true.

Explain that when adding by tens and ones, you split the second addend into two numbers which you add to the first addend. Remind students that a tens is a group of 10 and ones are the numbers from 1 to 9. Explain that you set the first addend at the start of the number line, and then move on the number line with the tens, followed by the ones of the second addend. If you go through a tens number, it is easier to first move to the next tens number, or the round number and then to jump with the rest of the second addend. Ask students what the total is of the given problem. Practice by adding with tens and ones on another number line once with the movement shown, and a second time where students determine which steps to take on the number line. Next, explain to students that you can add by tens and ones without a number line by splitting the second addend into tens and ones. You first add the tens of the second addend to the first addend. You then add the ones of the second addend to this number to find your total. Again, remind students that they can split the ones into two numbers to help them step to the next round number before adding the rest of the ones. (Example 68+2=70) Ask students which steps they take to calculate with different addition problems and ask students to calculate with tens and ones. Ask them to explain their thinking. Erase the grey boxes to show the answers.

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

- How do you add using tens and ones.

The students first practice calculating the total of an addition problem on the number line. Students are then show then steps taken on a number line but must add the total, finally students must add by tens and ones.

Discuss with students that it is important to be able to add to 100 using tens and ones, and being able to split the second addend into two parts because it will make it easier to add larger numbers. Ask students to determine which addition problem matches the number line shown. Students must then complete the addition problems shown on the interactive whiteboard. Drag the numbers to their correct places.

Students who have difficulty adding using tens and ones can make use of the number line. Ask them to calculate and draw on the number line the steps to calculate with tens and ones. Emphasize that they first jump with tens and then with ones. Show them that they can also take smaller steps with the ones to reach the next ten, before counting on.

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