Addition to 100 with one more or one less

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Students learn to add to 100 by adding with one more or one less to reach their total.

2.NBT.B.5

Discuss with students that it is useful to be able to add with one more or one less. If you know when you add one more or take one away, you don't need to redo calculations on sets of addition problems.

Play bingo with the students. Students write nine numbers on their cards with numbers from 70-100. You say a number and students are allowed to cross off that number, or its neighbor numbers on their bingo card. Say numbers until a students says "Bingo". Then show the flags on the board and ask which numbers are missing. Ask students to explain how they know which numbers are missing. Erase the grey boxes to show the correct answers.

Show the MAB-blocks on the interactive whiteboard. Say that there are 75 blocks and 5 more are added. Together they make 80. Discuss the other addition problems shown with the blocks. Ask students if they notice anything about the set of problems. They should notice that the answers are one more each time. Sometimes you don't have visual support. Sometimes you only see the numbers. Compare both problems. If the first addend is one more than the first addend in the second addition problem, then the answer of the second problem is also one more. Show students the addition problems on the interactive whiteboard and ask students to solve. Ask how they determined the answer of the second addition problem. Did they add all the numbers again? Or did they simply add one more? Say that when the second addend in the second addition problem is one more, that the answer is also one more. Practice the addition problems to clarify this.

Show the blocks and say that you have 30 blocks and you get 30 more. Your total is 60, Discuss the other addition problems shown with the blocks. Ask students if they notice anything about the set of problems. They should notice that the answers are one less each time. There aren't always blocks to count, sometimes you just have the numbers. Compare the addition problems. If the first addend in the second addition problem is one less than the first addend in the first addition problem, then the answer is also one less in the second addition problem. Show the addition problems on the interactive whiteboard and ask the students to solve. Ask how they determined the answer of the second addition problem. Did they add all the numbers again? Or did they simply take one away? Tell students that when the second addend of the second addition problem is one less than the second addend in the first addition problem that the answer is also one less. Practice the addition problems to clarify this. Finally show the problems on the interactive whiteboard and ask students to calculate the answers and write their answers on a sheet of paper. Have students hold up their answers to check their work.

Check that students are able to add to 100 with one more or one less by asking the following questions/giving the following exercises:

- What must you look at to know if you need to add one more or take one away?

- Calculate the following: 48+2= and 48+3 =

- Calculate the following: 10+63 =, 9+63 =, 8+63 =

Students first practice adding with one more or one less with sets of two addition problems. Students are then asked to solve sets of three addition problems.

Discuss with students that it is important to be able to add to 100 with one more or one less so you are able to easily solve sets of addition problems without calculating each one separately. Show the set of addition problems on the interactive whiteboard and ask students to calculate the answers. Ask how they solved the problems. Erase the grey boxes to reveal the answers.

Students who have difficulty with adding with one more or one less can be supported by using MAB blocks or manipulatives. Ask them to set out the addition problem of the first set and then to solve a problem with one more by adding one more block. Repeat this with different addition problems and then switch to demonstrating addition problems with one less.

Optional: MAB-blocks or manipulatives

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