standard_description: Understand subtraction as an unknown-addend problem. For example, subtract 10 – 8 by finding the number that makes 10 when added to 8.
problem_activity: Narrative
This Number Talk encourages students to think about equivalent forms of whole numbers and decomposing fractions in order to subtract. When students consider equivalent fractions, look for ways to decompose fractions, or use the structure of mixed numbers to find the value of each difference, they look for and make use of structure (MP7).
Launch
Display one expression.
“Give me a signal when you have an answer and can explain how you got it.”
Activity
1 minute: quiet think time
Record answers and strategy.
Keep expressions and work displayed.
Repeat with each expression.
Student Facing
Find the value of each expression mentally.
\(1 - \frac{8}{10}\)
\(1\frac{4}{10} - \frac{8}{10}\)
\(2\frac{4}{10} - \frac{8}{10}\)
\(10\frac{5}{10} - \frac{8}{10}\)
Student Response
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Activity Synthesis
“How are these expressions alike?” (They all involve subtracting
\(\frac{8}{10}\)
from a number that is at least 1. To subtract, it’s helpful or necessary to decompose a 1 or to write an equivalent fraction.)
“How did you use earlier expressions to help you find the value of later expressions?”


aligns: no