Topic C builds on students’ Grade 2 work with comparing numbers according to the value of digits in the hundreds, tens, and ones places (2.NBT.4). Lesson 12 formally introduces rounding two-digit numbers to the nearest ten. Rounding to the leftmost unit usually presents the least challenging type of estimate for students, and so here the sequence begins. Students measure two-digit intervals of minutes and metric measurements, and then use place value understanding to round. They understand that when moving to the right across the places in a number, the digits represent smaller units. Intervals of minutes and metric measurements provide natural contexts for estimation. The number line, presented vertically, provides a new perspective on a familiar tool.
Students continue to use the vertical number line in Lessons 13 and 14. Their confidence with this tool by the end of Topic C lays the foundation for further work in Grades 4 and 5 (4.NBT.3, 5.NBT.4). In Lesson 13, the inclusion of rounding three-digit numbers to the nearest ten adds new complexity to the previous day’s learning. Lesson 14 concludes the module as students round three- and four-digit numbers to the nearest hundred.