In Topic F, students begin adding like units within pairs of two-digit numbers. Lesson 23 focuses on taking interpretations of two-digit numbers a step further, having students interpret numbers such as 25 as 1 ten and 15 ones as well as 2 tens and 5 ones and as 25 ones. Working with this concept supports student understanding in the next lessons, when students add pairs such as 14 + 16 and initially make 2 tens and 10 ones. During Lessons 24 and 25, students interchangeably add sets of two-digit numbers where the ones digits produce a sum less than or equal to 10. For example, when adding 17 + 13, students decompose the second addend into 10 and 3. They then add 10 to 17, making 27, and then add the remaining ones. In Lesson 25, students also practice adding ones to the first addend and then adding the remaining ten. In Lesson 26 and 27, students add tens and ones when the ones digits have a sum greater than 10, such as 19 + 15. Students continue to decompose the second addend, alternating between adding on the ten first and making the next ten, as shown to the right. In Lesson 27, students solve the same problem using the varying strategies taught throughout the topic. Students continue to strengthen their use of Level 3 strategies for adding numbers to 40. The module closes with Lessons 28 and 29, wherein students solve problem sets of varied types to support flexibility in thinking as they add any pair of two-digits whose sum is within 40. In Lesson 29, students again share methods and representations for finding the sums.