Lesson 9

The purpose of this True or False is to elicit strategies and understandings students have for comparing two-digit numbers. These understandings will be helpful later when students will need to be able to determine numbers that are greater than or less than a given number.

• Display one statement.
• “Give me a signal when you know whether the statement is true and can explain how you know.”
• 1 minute: quiet think time

• Share and record answers and strategies.
• Repeat with each equation.

• “How can you explain your answers by describing the number of tens and ones on each side?”

The purpose of this activity is for students to use their understanding of place value to find a secret number in a number riddle. Initially, students are given clues that could have more than one secret number to encourage them to think about all the possibilities. Then, they receive a clue that allows them to figure out the secret number. Students will solve this type of riddle in small groups in the next activity.

• Groups of 3
• Give students access to connecting cubes in towers of 10 and singles.
• “We are going to solve number riddles. Let's do one together. I have chosen a secret number and will give you three clues to help you identify my number.”
• Display the clues in the student book.
• “Use the clues to identify numbers that could be my secret number. The clues aren’t in any specific order. You can use any clue to start.”

• 4 minutes: small-group work time
• Monitor for groups of students who started with different clues.
• “Does anyone know my secret number?” (It could be 34, 44 or 54 because they all fit the clue.)
• Display and read the new clue: “My number is the sum of 24 and 20.”
• “What is my secret number?”
• Share responses.

• “As we just saw, sometimes many different numbers fit the clues until you get the last clue. Keep that in mind as you solve Number Riddles with your group in the next activity.”

The purpose of this activity is for students to solve number riddles to find a secret number. Students work together to find the numbers that match the clues and justify why those numbers fit the clues (MP3). Then students read the last clue (shaded grey) and identify the secret number. Students rotate between riddles; it is also possible for students to remain in the same place and trade riddles.

• Create enough riddles from the Number Riddle Cards so each group of 2 students can have a different riddle. Fold the gray cards. Place each set of clues in a bag or envelope and label based on the letter of the riddle.

• Groups of 2
• Give each group an envelope with a riddle and access to connecting cubes in towers of 10 and singles.

• “You are going to solve number riddles. Each riddle has four clues. One clue is shaded and folded. Keep this clue until last. Read the other clues and work together to figure out which numbers fit the clues. Once you agree on what numbers fit all the clues, read the shaded, folded clue. Work together to find the answer and write your answer in your book. Then put all the clues back in the envelope and get ready to move on to the next riddle. I will let you know when to move.”
• 20 minutes: partner work time

• Invite students to share the secret number for each riddle.