options: Explain why addition and subtraction strategies work, using place value and the properties of operations. (Explanations may be supported by drawings or objects.)

Add up to four two-digit numbers using strategies based on place value and properties of operations.

Mentally add 10 or 100 to a given number 100–900, and mentally subtract 10 or 100 from a given number 100–900.

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

Add and subtract within 1000, 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. Understand that in 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.
problem_activity: Narrative
The purpose of this activity is for students to interpret and solve a story problem by adding or subtracting within 100. Students solve an Add To, Start Unknown problem, one of the more difficult problem types from grade 1. Students begin the activity by looking at the problem displayed, rather than in their books. At the end of the launch, students open their books and work on the problem.
Monitor for students who use methods that show adding or subtracting by place to share in the synthesis. Students who choose to use connecting cubes or base-ten blocks or who draw a diagram to represent the situation are using tools strategically (MP5). During the synthesis, invite all students to explain why these methods work using their understanding of place value.
This activity uses
MLR6 Three Reads
. Advances: reading, listening, representing. Some students may benefit from continued use of MLR6 to support reading comprehension in Activity 2.
Required Materials
Materials to Gather
Base-ten blocks
Connecting cubes
Launch
Groups of 2
Give students access to connecting cubes and base-ten blocks.
“Have you ever been on a field trip? Where did you go?”
“Did everyone on your field trip stay together the whole time or did you split into smaller groups?”
MLR6 Three Reads
Display only the story problem, without revealing the question.
“We are going to read this problem 3 times.”
1st Read: “Some students were on the bus to go to the zoo. Then 34 more students got on. Now there are 55 students on the bus.”
“What is this story about?”
1 minute: partner discussion.
Listen for and clarify any questions about the context.
2nd Read: “Some students were on the bus to go to the zoo. Then 34 more students got on. Now there are 55 students on the bus.”
“What are all the things we can count in this story?” (number of students who started the story on the bus, number of students who got on next, the total number of students on the bus, the number of buses)
30 seconds: quiet think time
2 minutes: partner discussion
Share and record all quantities.
Reveal the question.
3rd Read: Read the entire problem, including the question aloud.
“What are different ways we can solve this problem?” (We could subtract the number of students who got on the bus second from the total. We could add to the number of students who got on the bus until we get to the total.)
30 seconds: quiet think time
1–2 minutes: partner discussion
Activity
5 minutes: independent work time
Monitor for students who:
use base-ten blocks or base-ten diagrams to show adding tens to tens or ones to ones
use base-ten blocks or base-ten diagrams to show subtracting from tens or ones from ones
Student Facing
Some students were waiting on the bus to go to the zoo. Then 34 more students got on. Now there are 55 students on the bus. How many students were on the bus at first?
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Student Response
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Activity Synthesis
Invite previously identified students to share.
Record student methods using equations.
“How are these methods the same?” (They both show tens and ones. They both thought of keeping the tens with the tens and ones with the ones.)


correct_option_index: 3