In a previous unit, students added within 20, including adding one-digit numbers and teen numbers. Students also added multiples of 10 and two-digit numbers within 100, using place value reasoning. In this lesson, students think about how two-digit numbers change when they add only ones or only tens. For example, in 24 + 3 = 27 students notice that the digit in the ones place changes when 3 is added to 24, but the digit in the tens place does not. In 24 + 30 = 54, students notice that the digit in the tens place changes, when 30 is added to 24, but the digit in the ones place does not. This lesson provides an opportunity for teachers to formatively assess student’s work with addition from previous units.

How does the work of this lesson prepare students for adding 2 two-digit numbers in the next lesson?