In this activity, students learn Stage 4 of the What’s Behind My Back? center. In this new stage, students work with 20 connecting cubes, organized into two towers of 10 cubes. One partner snaps the tower and puts one part behind their back and shows the other part to their partner. The other partner figures out how many cubes are behind their partner’s back. Students record an addition equation with a blank to represent the missing cubes. Students may write equations with the blank as the first or second addend. Ask students to explain what each number and blank in the equation represents in the context of the center activity (MP2).

• Display 9 cubes.
• “¿Qué ecuación de suma puedo escribir para representar el número de cubos que ustedes conocen y el número de cubos que tienen que descubrir?” // “What’s an addition equation I can write to represent the number of cubes you know and the number of cubes you need to figure out?” (9 + _____ = 20)
• “Cuéntenle a su compañero cómo pueden saber cuántos cubos faltan” // “Tell your partner how you can figure out how many cubes are missing.”
• Monitor for students who talk about making a 10 and knowing there is one more 10.
• Share responses.

The purpose of this activity is for students to find the value that makes an addition or subtraction equation true, with totals of 20. Students may use whatever method makes sense to them. In the Launch, students work with a new partner and fill in the unknown addend in the equations on their recording sheet from the first activity.

• Invite selected students to show their thinking for and .
• “Para encontrar la respuesta, ¿cómo les ayudó pensar sobre decenas?” // “How did thinking about a ten help them find their answer?” (They knew 20 is 2 tens, so they used 1 of the tens to make the problems easier.)