standard_description: Mentally add 10 or 100 to a given number 100–900, and mentally subtract 10 or 100 from a given number 100–900.
problem_activity: Narrative
The purpose of this activity is for students to learn stage 2 of the Mystery Number center. Students pick 3 cards and make a mystery three-digit number. Students give clues based on the sentence starters and their partner tries to guess their number. Students should remove cards that show 10 from their deck. When they give clues and guess the number, students demonstrate understanding of the base-ten system, both the  meaning of each digit in a number and the order of the numbers (MP7).
Required Materials
Materials to Gather
Number cards 0–10
Materials to Copy
Mystery Number Stage 2 Directions
Launch
Groups of 2
Give each group a set of number cards and directions.
“We are going to learn a new center called Mystery Number that you might have played in first grade. You will pick 3 cards and make a mystery three-digit number. Then you will give your partner clues and they will try to guess your mystery number.”
“Read the directions with your partner to see if you have any questions.”
Answer any questions or play a round against the class to demonstrate the center.
Activity
15 minutes: partner work time
Monitor for clues to share during the synthesis.
Activity Synthesis
“What types of clues were particularly helpful to you when you were trying to guess your partner’s number?”


aligns: no