Lesson 18

The purpose of this warm-up is to introduce equations with a symbol for the unknown value. While students may notice and wonder many things about this equation, how the equation relates to the image is the important discussion point. When students notice that the unknown value represents a specific quantity within the image, they reason abstractly and quantitatively (MP2).

• Groups of 2
• Display the image.
• “¿Qué observan? ¿Qué se preguntan?” // “What do you notice? What do you wonder?”
• 1 minute: quiet think time

• “Discutan con su compañero lo que pensaron” // “Discuss your thinking with your partner.”
• 1 minute: partner discussion
• Share and record responses. 

• “¿Cómo corresponde a la imagen la ecuación que tiene el valor desconocido?” // “How does the equation with the unknown value match the picture?” (There are 4 basketballs in the open bag and there are some more things in the other bag but we don't know how many. There must be 10 altogether.)

The purpose of this activity is for students to match story problems to equations with a symbol for the unknown (MP2). Each equation is written to match the way the numbers are presented in the story problem. Problem G has more than one equation, which prompts students to discuss the relationship between addition and subtraction (MP7). During the synthesis, students discuss how an equation with a symbol for the unknown matches a Take From, Result Unknown story problem.

• Create a set of Story Problem Cards and Equation Cards for each group of 2.

• Groups of 2
• Give each group a set of both cards.

• “Tienen dos grupos de tarjetas. Un grupo de tarjetas tiene los problemas-historia que usamos en la última lección. El otro grupo de tarjetas tiene ecuaciones con valores desconocidos” // “You have two sets of cards. One set of cards has the story problems we used in the last lesson. The other set of cards has equations with unknown values.”
• “Trabajen con su compañero para emparejar los problemas-historia con las ecuaciones. Una historia tiene más de una ecuación. Asegúrense de que pueden explicar cómo saben que corresponden” // “Work with your partner to match the story problems to the equations. One story has more than one equation. Be sure you can explain how you know they match.”
• 10 minutes: partner work time

• “¿Cuál ecuación corresponde a la tarjeta C? ¿Cómo lo saben?” // “Which equation matches Card C? How do you know?” (\(9 - 6 = \boxed{\phantom{3}}\). 9 represents how many students were sliding. 6 represents how many students leave so that is \(9 - 6\). The box represents how many are left, which is the answer to the problem.)
• Repeat for problems F and H.
• Display equation cards 6 and 8.
• “¿Qué observan en estas ecuaciones?” // "What do you notice about these equations?" (They both have a total of 9 and one part is 4. The other part is the unknown. They both match problem G.)
• “¿Cómo corresponde cada una de estas ecuaciones al problema-historia?” // “How does each of these equations match the story problem?” (There are 9 students jumping Double Dutch and 4 students jumping on their own. I need to find the difference, so I can subtract \(9 - 4\) to find the answer or I can say that \(9 = 4 + \boxed{\phantom{5}}\). 9 equals 4 plus some more students.)

The purpose of this activity is for students to interpret two different equations with a symbol for the unknown in relation to a story problem. Students are presented with a Put Together, Addend Unknown story problem and two equations that match it, which allows students to further explore the relationship between addition and subtraction. Students explain how each equation matches the story problem and make connections between them. When students interpret different equations in terms of a story problem, they model with mathematics (MP4).

• Groups of 2
• Give students access to connecting cubes or two-color counters.
• “Vamos a examinar una historia más y dos ecuaciones que le corresponden. Esta historia es sobre unos estudiantes que juegan un juego que se llama bingo” // “We are going to look at one more story and two equations that match. This story is about students playing the game called bingo.”

• Read the task statement. 
• 5 minutes: independent work time
• 4 minutes: partner discussion
• Monitor for students who create a clear drawing and can use it to explain how each equation matches the story.

• Invite previously identified students to share.
• “¿Cómo se relacionan las ecuaciones entre sí?” // “How do the equations relate to each other?” (The second equation can be used to find the missing number in the first equation. The number that goes in the box is the same.)