To deepen the understanding of subtraction as the difference between two numbers, have your students work through these problems one at a time. As you present each problem, provide students with some quiet, individual think time during which they can solve the problem and show their thinking on a dry erase board, paper, etc. Then have students share their thinking either in pairs or with the whole group.  As you facilitate these conversations, emphasize that the Round the Subtrahend strategy is efficient because the subtrahend is just under the decade.

The problems are supporting the move towards subtracting numbers mentally and creating number sense. Students should be encouraged to do parts of the subtracting in their head. For this purpose students need to develop a mental number line. 

The last two prompts are stand alone without the rounded subtrahend given. Can students round to the appropriate number on their own?

String of Prompts:

String 1:

23 – 10

23 – 9

String 2:

36 – 10

36 – 9

String 3:

44 – 20

44 – 19

String 4:

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String 5:

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