The purpose of this activity is for students to apply their place value understanding to add an amount of tens or ones to a two-digit number. Students also use place value reasoning to determine whether a number of tens or ones was added to a two-digit number. Throughout the activity, students explain how they add and how they determined the unknown addend with an emphasis on place value vocabulary (MP3, MP6).

• “El compañero de Priya giró la ruleta y esta cayó en 34. Priya escogió un 5. Si ella quiere sumar 5 unidades, ¿cómo puede encontrar la suma?” // “Priya’s partner landed on 34 on the spinner. Priya picked a 5. If she wants to add 5 ones, how could she find the sum?“ (She could start at 34 and count on 35, 36, 37, 38, 39. She could add 5 more ones to the 4 ones in 34.  so its 39.)
• “¿Cómo puede encontrar la suma si quiere sumar 5 decenas?” // “How can she find the sum if she wants to add 5 tens?“ (She could count on by tens: 44, 54, 64, 74, 84. She could add 3 tens and 5 tens to get 8 tens.)

In this activity, students add a one-digit number or a multiple of 10 and a two-digit number, without composing a ten. The order of the problems encourages students to analyze the difference between adding ones or tens (adding 5 or adding 50), which builds on the previous activity. Students rely on methods that they have learned such as counting on or using known facts to add. In the Activity Synthesis, students may say they notice that only the digit in the ones place of a two-digit number changes when they add a one-digit number to it. While this statement is true about the numbers in these problems, it will not be true when students add in future work. It may be helpful to record this conjecture on chart paper and revisit it again in future lessons. This will give students an opportunity to explain whether or not it is always true. 

• “En cada caso, ¿cómo encontraste el número que hace que la ecuación sea verdadera?” // “How did you find the number that makes each equation true?”
• “¿Cómo se vería cada expresión si usáramos cubos encajables? ¿Habría el mismo número de cubos?” // “How would each expression look with connecting cubes? Would there be the same number of cubes?”

• Display the first 3 problems.
• “¿Qué observaron sobre las ecuaciones y las sumas?” // “What did you notice about the equations and the sums?” (It was like adding the same number, just to a different place. When I add , I only added the numbers in the ones place. When I add , I added 5 tens and 0 ones to 43. The number in the ones place stayed the same.)