Similar to Topic E’s addition methods, Topic I allows students to learn methods for subtraction which involve subtracting 0 and 1, subtracting the whole number, subtracting one less than the whole number, and using familiar decompositions (5-groups and partners of 10) to conceptualize subtraction as finding a missing part (1.OA.6). In Lesson 33, students use pictures and simple math drawings to show 0 less and 1 less and construct number sentences (1.OA.5). The discussion in Lesson 34 around what happens each time we take away 0 or 1 with numbers within 10 leads students to an understanding that this same reality remains true with all numbers. Similarly, students explore what happens in both n-n and n-(n-1) situations. They notice, “When I take 5 away from 5, I have zero every time!” and “5 – 4 is 1, just like 6 – 5 is 1! It’s just a number minus a number that’s one less gives me 1!” and again generalize this new understanding. In Lesson 35, students transfer their knowledge of both doubles and fives to the context of subtraction, where they extract those known facts from given expressions. For instance, when faced with 8 – 5, students access the decomposition of 8 (“I know that 5 and 3 makes 8!”), and apply that understanding to help them solve subtraction problems (“So 8 – 5 must be 3!”). Lessons 36 and 37 continue on this explicit decomposition and subtraction connection, as students use their knowledge of partners of 10 and partners of 9 to help them solve subtraction stories and equations efficiently. Topic I is full of students using strategies, and discussing those strategies and patterns in order to gain fluency and facility with subtraction within 10, and ultimately beyond!