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