CCSS.Math.Content.1.NBT.C.6
The standard
Subtract multiples of 10 in the range 10-90 from multiples of 10 in the range 10-90 (positive or zero differences), using concrete models or drawings and strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; relate the strategy to a written method and explain the reasoning used.
Common Core State Standards for Mathematics · Number and Operations in Base Ten
What this standard means
Students need to subtract tens from tens, like 70 minus 30, and explain it as tens being taken away, not as single ones. They should use tools, drawings, or place value thinking, then connect that work to a written number sentence.
Mastery looks like a student saying, “7 tens minus 3 tens is 4 tens, so 70 minus 30 is 40,” and showing it with blocks or a drawing. Students often get stuck treating the zeroes as extra steps, counting back by ones, or forgetting that the answer must still be a multiple of 10.
Ways to teach it
- Give students base-ten rods and have them build 80, remove 50, then write 80 minus 50 equals 30.
- Ask students to explain in writing: How is 60 minus 20 like 6 minus 2?
- Show four problems, including 90 minus 40, and have students draw tens rods to solve each on a whiteboard.
- Use pretend classroom supply boxes with 10 pencils each, then ask how many pencils remain after giving away some boxes.
Plan a lesson for CCSS.Math.Content.1.NBT.C.6
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