Maryland Math 2.NOS.C.8

2.NOS.C.8 is a cluster heading. These are the individual standards under it.

• 2.NOS.C.8.a

  Use strategies based on counting (e.g., counting on, counting back) and place value (e.g., partial sums, making tens, etc.) to add and subtract.

• 2.NOS.C.8.b

  Use properties of operations, and the inverse relationship between addition and subtraction to add and subtract.

• 2.NOS.C.8.c

  Determine and explain when a strategy is most efficient.

01

What Students Need to Do

Students add and subtract within 100 using counting, tens and ones, number properties, and related facts. They choose a method that fits the numbers and explain why it works well.

02

What Mastery Looks Like

A student solves 48 + 7 by making 50, then adding 5. They solve 63 - 28 by counting up from 28 to 63 and check that 28 + 35 = 63. They can explain why these methods fit the numbers.

03

Common Misconceptions

Some students treat 46 + 20 as 46 + 2 and answer 48. Others answer 45 for 63 - 28 by subtracting the smaller digit from the larger digit in each place. Some think they can swap the numbers in a subtraction problem.

04

How to Assess It

Give students 48 + 7 and 63 - 28 as an exit ticket. Ask them to solve each, explain why they chose their method, and check the subtraction with addition.

1. 01

   Build 54 - 28 with base-ten blocks, trade one ten for ten ones, and check the difference by adding 28.

2. 02

   Write 48 + 7 on the board and ask, 'Would you count on or make 50? Why?'

3. 03

   Draw two cards numbered 10 to 50, subtract the smaller from the larger, and score a point for checking with addition.

4. 04

   Tell students 64 library books were on a shelf and 27 were checked out; find how many remain and check by adding.

• 6.NOS.B.3

  Fluently multiply and divide multi-digit decimals to the thousandths in context.

• 4.NOS.B.3

  Fluently add and subtract multi-digit whole numbers within 1,000,000.

• 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 ...

• 3.NOS.D.7

  Fluently add and subtract within 1,000.