In Topic B, students practice counting by ones and skip-counting by tens and hundreds. They start off with simple counting by ones and tens from 90 to 124 and 124 to 220. They then count by ones, tens, and hundreds from 200 to 432 and from 432 to 1,000 (2.NBT.2). They apply their new counting strategies to solve a change unknown word problem (2.OA.1). “Kinnear decided that he would bike 100 miles this year. If he has biked 64 miles so far, how much farther does he have to bike?” In counting students make use of the structure provided by multiples of ten and a hundred. Students think in terms of getting to a ten or getting to a hundred. They also identify whether ones, tens, or hundreds are the appropriate unit to count efficiently and effectively. Making this determination requires knowing and understanding structures, similar to knowing the ground on which you are going to build a house and with what materials you want to build.