# 1.NBT.C.4

Add within 100, including adding a two-digit number and a one-digit number, and adding a two-digit number and a multiple of 10, 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. Although this standard involves only adding a two-digit number with a muliple of ten, students will understand that in adding two-digit numbers, one adds tens and tens, ones and ones; and sometimes it is necessary to compose a ten.

## Understanding the Standard:

• Provide students with problems to perform the following operations: 1. add a two-digit number and a one-digit number; 2. add a two-digit number and a multiple of ten; 3. add two two-digit numbers. (Students can initially be given addition problems that do not require regrouping, and then addition problems that do require composing a ten.)
• Provide students with a variety of manipulatives and technologies which can aid in their practice of addition and subtraction through 100.
• Base-10 manipulatives can be used to show students how to concretely model addition and to reinforce the idea of place value, of adding tens and tens, ones and ones, and when necessary, composing a ten.

## Specific Questions to Focus Instruction:

• Can students add and subtract two-digit numbers?
• Are students able to demonstrate mastery of place value by adding only those digits in the ones positions and then adding only those digits in the tens position (if regrouping is not necessary)? When necessary, can students compose a ten?
• Can students add a two-digit number and a one-digit number?
• Can students add a two-digit number and a multiple of ten?
• Can students use different concrete models to add, and can they relate these strategies to a written method of addition?
• Can students explain the reasoning behind the strategy used?

## Skills

Prior to: Students are able to show mastery in adding and subtracting one-digit numbers.

At Grade Level: Students can add and subtract two-digit numbers through 100, showing an understanding of the value each digit possesses.

Moving Beyond: Students will build upon prior knowledge to explore the concept of regrouping in addition and subtraction with two-digit numbers. Further, they will begin to add and subtract three-digit numbers, first with no regrouping and then moving on to regrouping in a variety of positions. Go to 2.NBT.B.7 to see the progression of related skills.