standard_description: Given a two-digit number, mentally find 10 more or 10 less than the number, without having to count; explain the reasoning used.
problem_activity: Narrative
This Number Talk encourages students to think about how to use the addition and subtraction facts they know and their understanding of place value to mentally find values of expressions. The strategies elicited here will be helpful as students use the facts they know to develop fluency with addition and subtraction within 100.
Students may notice that calculating
\(6 - 3\)
is repeated in each expression and use this to mentally find the values (MP8). Students must also attend to precision in how they describe how they use known facts or other methods (MP6), especially when they describe subtracting by place. Encourage students to tell more about their strategy if they use unclear language to describe their method and to reflect on how their description could be interpreted differently without more precise language. For example, if students say they knew
\(66-30=36\)
because they know
\(6-3\)
, ask students to explain how that fact helped them subtract.
Launch
Display one expression.
“Give me a signal when you have an answer and can explain how you got it.”
1 minute: quiet think time
Activity
Record answers and strategy.
Keep expressions and work displayed.
Repeat with each expression.
Student Facing
Find the value of each expression mentally.
\(6 - 3\)
\(66 - 3\)
\(66 - 30\)
\(66 - 33\)
Student Response
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Activity Synthesis
“How is each expression like
\(6 - 3\)
? How are the expressions different?” (Each expression has the digits 6 and 3. In each expression you could use what you know about
\(6 - 3\)
. Some expressions you are finding 6 ones
\(-\)
3 ones. In some expressions you are finding 6 tens
\(-\)
3 tens.)


aligns: no