Lesson 25

The purpose of this Choral Count is for students to practice counting by one starting at a number other than one. These understandings help students make the transition from counting all to counting on when adding and subtracting.

• “Cuenten de 1 en 1, empezando en 50” // “Count by 1, starting at 50.”
• Record as students count.
• Stop counting and recording at 100.

• “¿Qué patrones ven?” // “What patterns do you see?”
• 1-2 minutes: quiet think time
• Record responses.

• “¿Qué número va después de 79? ¿Qué número va antes de 50?” // "What number comes after 79? What number comes before 50?"

The purpose of this activity is for students to find differences using methods they choose. As students work, they may feel more comfortable with one method than another. The numbers were chosen to encourage different methods (counting on and taking away) so students can consider the numbers in a specific expression as they find the difference. During the activity synthesis, students share which method they used for a specific problem and why they chose it (MP3).

• Groups of 2
• Give students access to double 10-frames and connecting cubes or two-color counters.

• “Hemos estado aprendiendo sobre diferentes métodos para restar. Observen cada ecuación y piensen qué método quieren usar para encontrar el número desconocido. Pueden usar métodos diferentes para cada problema” // “We have been learning about different methods for subtracting. Look at each equation and think about which method you want to use to find the missing number. You can use different methods for each problem if you would like.”
• 8 minutes: independent work time
• As students work, consider asking:
  • “¿Cuál método usaron?” // “Which method did you use?”
  • “¿Por qué escogieron ese método para esta ecuación?” // “Why did you choose that method for this equation?”
  • “¿Usaron este método para todas las ecuaciones? ¿Por qué sí o por qué no?” // “Do you use this method for every equation? Why or why not?”
• Monitor for students who take away three for \(19 - 3 = \boxed{\phantom{\f{4}}}\), and can explain the benefit to choosing this method for these numbers. 
• 4 minutes: partner discussion

• Invite previously identified students to share.
• “Podemos escoger qué método usar en cada ecuación dependiendo de los números que aparecen. Si hay que quitar mucho, podríamos, más bien, contar hacia adelante. Pero si hay que quitar poco, podemos simplemente usar el método de quitar. Ambos métodos sirven, pero podemos pensar en cuál es más eficiente para cada ecuación” // “We can choose to use different methods for different equations depending on the numbers. If we need to take away a lot, we might choose to count on instead. If we only need to take away a few, we might choose to take away. Both methods will work, but we can think about which method is more efficient for each equation.”

The purpose of this activity is for students to solve different types of story problems involving teen numbers. Each story problem can be solved using either addition or subtraction. Students may use either operation for the second and third problems and any method that makes sense to them. They write equations to match the stories and should be able to explain how they match (MP2).

• Groups of 4
• Give students access to double 10-frames and two-color counters or connecting cubes.

• Read the task statement.
• 6 minutes: independent work time
• Monitor for students who solve Kiran’s story problem by:
  • Counting on from 9
  • Taking away 9
  • Subtracting from the ten or subtracting to get to 10
• “Compartan con su grupo cómo pensaron” // “Share your thinking with your group.”
• 4 minutes: small-group discussion

• Display Kiran’s story problem.
• Invite previous identified students to share their methods and equations.
• “¿De qué manera sus ecuaciones corresponden a su trabajo? ¿En qué parte de la ecuación vemos la respuesta a la pregunta?” // “How do their equations match their work? Where do we see the answer to the question in their equation?”