Maryland Math 2.NOS.C.9.a
The Standard
- 2.NOS.C.9
- Add and subtract within 1000, recognizing that when adding or subtracting three-digit numbers, one adds or subtracts hundreds and hundreds, tens and tens, ones and ones; and sometimes it is necessary to compose or decompose tens or hundreds.
Use concrete models or drawings to add and subtract.
Maryland College and Career Ready Standards for Mathematics (2025) · USE REPRESENTATIONS, PLACE VALUE UNDERSTANDING, AND PROPERTIES OF OPERATIONS TO ADD AND SUBTRACT.
Teacher's field guide
What This Standard Means
What Students Need to Do
- Students build or draw numbers with tens and ones. They combine or take away parts, trading a ten for ten ones when needed. They use the model to explain their answer.
What Mastery Looks Like
- A student can show 34 + 28 with tens and ones, trade ten ones for a ten, and get 62. For 52 - 17, they can trade one ten for ten ones, take away 17, and get 35.
Common Misconceptions
- Some students count a bundle of ten as one. Others write 52 - 17 = 45 by subtracting 2 from 7. When trading a ten, they may forget to remove it from the tens.
How to Assess It
- Exit ticket: Draw tens and ones to solve 43 - 18. Circle where you traded one ten for ten ones, and write the answer.
Lesson moves
Ways to Teach It
Give pairs bundled craft sticks and singles to build 34 + 28, trade ten singles for a bundle, and record the sum.
Ask students to sketch 52 - 17, then tell a partner why they traded one ten for ten ones.
Play Draw and Trade: students draw two number cards, build their sum with base-ten blocks, then check it with a sketch.
Write '45 tickets, 18 sold' on the board; students sketch tens and ones to find how many tickets remain.
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Printable 2.NOS.C.9.a Worksheet
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