Lesson 18: A Lot of Fractions to Add

A Lot of Fractions to Add

10 min

Narrative

The purpose of this Number Talk is to encourage students to apply properties of operations (especially the commutative and associative properties of addition) to mentally find sums of three or more whole numbers. The reasoning elicited here will be helpful later in the lesson when students add three or more tenths and hundredths.

To mentally add several two- and three-digit numbers, students look for and make use of structure (MP7), such as finding pairs of numbers that add up to 10 or 100, or that end in 0 or 5.

Launch

Display one expression.
“Give me a signal when you have an answer and can explain how you got it.”
1 minute: quiet think time

Teacher Instructions

Record answers and strategies.
Keep expressions and work displayed.
Repeat with each expression.

Student Task

Find the value of each expression mentally.

$54 + 2 + 18$
$61 + 104 + 39$
$25 + 63 + 75 + 7$
$50 + 106 + 19 + 101$

Sample Response

Sample response:

74: $18 + 2$ is 20, and $54 + 20$ is 74.
204: $61 + 39$ is 100, and $104 + 100$ is 204.
170: $25 + 75$ is 100, and $63 + 7$ is 70.
276: $101 + 19$ is 120, $120 + 50$ is 170, and $170 + 106$ is 276.

Activity Synthesis (Teacher Notes)

“What strategies were helpful for adding multiple numbers?” (Find two or three numbers that add up to 10 or 100, add numbers that end with 5 or 0 first, add familiar numbers first, and put the numbers into groups of two, and then add what’s in each group before adding the groups.)
Consider asking:
- “Who can restate _______ 's reasoning in a different way?”
- “Did anyone have the same strategy but would explain it differently?”
- “Did anyone approach the expression in a different way?”
- “Does anyone want to add on to____’s strategy?”

Standards

Building On

3.NBT.2·Fluently add and subtract within 1000 using strategies and algorithms based on place value, properties of operations, and/or the relationship between addition and subtraction.
3.NBT.A.2·Fluently add and subtract within 1000 using strategies and algorithms based on place value, properties of operations, and/or the relationship between addition and subtraction.

25 min

10 min

Knowledge Components

All skills for this lesson

No KCs tagged for this lesson

A Lot of Fractions to Add

10 min

Narrative

To mentally add several two- and three-digit numbers, students look for and make use of structure (MP7), such as finding pairs of numbers that add up to 10 or 100, or that end in 0 or 5.

Launch

Display one expression.
“Give me a signal when you have an answer and can explain how you got it.”
1 minute: quiet think time

Teacher Instructions

Record answers and strategies.
Keep expressions and work displayed.
Repeat with each expression.

Student Task

Find the value of each expression mentally.

$54 + 2 + 18$
$61 + 104 + 39$
$25 + 63 + 75 + 7$
$50 + 106 + 19 + 101$

Sample Response

Sample response:

74: $18 + 2$ is 20, and $54 + 20$ is 74.
204: $61 + 39$ is 100, and $104 + 100$ is 204.
170: $25 + 75$ is 100, and $63 + 7$ is 70.
276: $101 + 19$ is 120, $120 + 50$ is 170, and $170 + 106$ is 276.

Activity Synthesis (Teacher Notes)

“What strategies were helpful for adding multiple numbers?” (Find two or three numbers that add up to 10 or 100, add numbers that end with 5 or 0 first, add familiar numbers first, and put the numbers into groups of two, and then add what’s in each group before adding the groups.)
Consider asking:
- “Who can restate _______ 's reasoning in a different way?”
- “Did anyone have the same strategy but would explain it differently?”
- “Did anyone approach the expression in a different way?”
- “Does anyone want to add on to____’s strategy?”

Standards

Building On

3.NBT.2·Fluently add and subtract within 1000 using strategies and algorithms based on place value, properties of operations, and/or the relationship between addition and subtraction.
3.NBT.A.2·Fluently add and subtract within 1000 using strategies and algorithms based on place value, properties of operations, and/or the relationship between addition and subtraction.

A Lot of Fractions to Add

Warm-upNumber Talk: A Bunch of Numbers10 minKCs

Narrative

Launch

Student Task

Sample Response

ActivityStack Centavos and Pesos25 minKCs

ActivityMore than Two Fractions10 minKCs

Knowledge Components

A Lot of Fractions to Add

Warm-upNumber Talk: A Bunch of Numbers10 minKCs

Narrative

Launch

Student Task

Sample Response

ActivityStack Centavos and Pesos25 minKCs

ActivityMore than Two Fractions10 minKCs

10 min

25 min

10 min

10 min

25 min

10 min