# 2.NBT.B.5*

Fluently add and subtract within 100 using strategies based on place value, properties of operations, and/or the relationship between addition and subtraction.

*TNCore Focus Standard

## Summary:

Students have developed a firm understanding of place value of two-digit numbers and to subtract multiples of ten, and are ready to add and subtract within 100 (including the case of adding or subtracting a two-digit number and a one-digit number, and two two-digit numbers). First students are given problems where regrouping is not necessary, and later, problems where regrouping is necessary. Further, students understand that in addition and subtraction, digits in the ones place are added and subtracted; digits in the tens place are added and subtracted; and sometimes regrouping is necessary. In addition, sometimes we must regroup ten ones to form an additional ten, and in subtraction, sometimes we must break a ten into ten ones. The eventual goal of this standard is flunecy. This will not happen all at once; students will build gradually towards having procedures and strategies by which they can fluently add an subtract, including standard algorithms and skip-counting up or down.

## Understanding the Standard:

• In a given addition and subtraction problems, ask students to identify which digits are in the one and tens positions. In addition, they should be able to identify the digits in the ones and tens positions.
• Provide students with a variety of manipulatives and technologies (such as base-ten blocks or drawings) which can aid in their practice of addition and subtraction through 100.
• Use properties of addition to make addition more fluid. For example, 64 + 8 can be thought of as (62 + 2) + 8, reordering addends allows (2 + 62) + 8, regrouping addends yields 2 + (62 + 8) which gives 2 + 70, which is equal to 72. This is what is meant by using a strategy with the properties of the operations.  Another strategy would be using skip-counting for addition (e.g., 58 + 15 can be found by skip-counting up from 58 by ten, and then by five. An additional strategy is to break down place value: 58 + 15 can be thought of as 50 + 10 + 8 + 5, whcih is 60 + 13, or 73.
• Students should have at least one algorithm in place that is robust and works in all cases, but they also should be encouraged to use alternative strategies if they can do so quickly and accurately.

## Questions to Focus Instruction:

• Can students add and subtract multiples of ten fast and accurately, and explain the strategy used?
• Can students add and subtract within 100 without only counting, but using strategies based on place value, properties of operations and the relationship between addition and subtraction?
• Can students explain the reasoning behind the strategy used?
• Are students able to demonstrate mastery of place-value-based algorithms for addition and subtraction by adding and subtracting those digits in the ones positions, then by adding or subtracting those digits in the tens position by regrouping tens or ones as appropriate?
• Can students add and subtract numbers within 100 quickly and accurately?
• ## Skills

Prior to:
Students can add two-digit numbers through 100, showing an understanding of the value each digit possesses.