The purpose of this activity is for students to add multiples of 10 and two-digit numbers in a way that makes sense to them. Students may use connecting cubes in towers of 10 and singles. They represent their thinking using drawings, numbers, or words. Students may make base-ten drawings to show the addition. Students may find the sum mentally, applying what they have learned about place value in previous lessons. Students may also use both methods, depending on the numbers in the problem.

• Invite previously identified students to share.
• “¿En qué se parecen estos métodos? ¿En qué son diferentes?” // “How are these methods alike? How are they different?” (They all added tens to tens. Some students started with the first number. Some students started with the second number.)

The purpose of this activity is for students to use their understanding of place value to make an equation true when a digit in a two-digit number is “unknown.” Students consider the values of the digits in order to justify their thinking. Students may use connecting cubes in towers of 10 and singles, create drawings, or write about their reasoning.

• Invite students to share how they determined the unknown digit in the first equation.
• Invite students to share how they know the second equation can’t be true.