CCSS.Math.Content.1.NBT.C.5
The standard
Given a two-digit number, mentally find 10 more or 10 less than the number, without having to count; explain the reasoning used.
Common Core State Standards for Mathematics · Number and Operations in Base Ten
What this standard means
Students need to see a two-digit number as tens and ones. They should change only the tens digit when adding or subtracting ten, while the ones stay the same. They also need to explain it with words, base-ten blocks, drawings, or a hundred chart.
Mastery sounds like, “46 has 4 tens and 6 ones. Ten more is 5 tens and 6 ones, so 56.” Students often count by ones, change the ones digit, or get confused when subtracting ten from numbers in the teens. A hundred chart helps, but they should move toward mental reasoning.
Ways to teach it
- Hands-on activity: Give students base-ten rods and ones cubes, build 34, then add or remove one rod and say the new number.
- Prompt: Ask, “How do you know 10 less than 72 is 62 without counting?” and have students draw or write their reasoning.
- Quick assessment: Show five numbers on cards, and students write 10 more and 10 less on mini whiteboards.
- Real-world connection: Use classroom book bins numbered 18, 28, 38, and ask which bin is ten more or ten less.
Plan a lesson for CCSS.Math.Content.1.NBT.C.5
Generate a complete lesson plan aligned to this standard, with objectives, activities, and materials. Free, no account needed.
Related standards
- CCSS.Math.Content.K.OA.A.4
For any number from 1 to 9, find the number that makes 10 when added to the given number, e.g., by using objects or drawings, and record the answer with a drawi...
- CCSS.Math.Content.2.NBT.B.8
Mentally add 10 or 100 to a given number 100—900, and mentally subtract 10 or 100 from a given number 100—900.
- CCSS.Math.Content.5.NBT.A.1
Recognize that in a multi-digit number, a digit in one place represents 10 times as much as it represents in the place to its right and 1/10 of what it represen...
- CCSS.Math.Content.1.NBT.C.6
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 strate...