Grade 2 Mathematics Module 5, Topic C

boy with chalkboard

Topic C builds upon Module 4’s groundwork, now decomposing tens and hundreds within 1,000 (2.NBT.7). In Lesson 13, students model decompositions with number disks on their place value charts while simultaneously recording these changes in the written vertical form. Students draw a magnifying glass around the minuend, as they did in Module 4. They then ask the familiar questions: Do I have enough ones to subtract? Do I have enough tens? When the answer is no, students exchange one of the larger units for ten of the smaller units. They record the change in the algorithm, following this procedure for each place on the place value chart. In Lessons 14 and 15, students transition to making math drawings, thus completing the move from concrete to pictorial representations. They follow the same procedure for decomposing numbers as in Lesson 13, but now they use number disk drawings (Lesson 14) and chip models (Lesson 15). Students continue to record changes in the vertical method as they relate their drawings to the algorithm, and they use place value reasoning and the properties of operations to solve problems with up to two decompositions (e.g., 547 – 168, as shown above). Lessons 16 and 17 focus on the special case of subtracting from multiples of 100 and numbers with zero in the tens place. Students recall the decomposition of 100 and 200 in Module 4 in one or two steps, using the same reasoning to subtract from larger numbers. For example, 300 can be decomposed into 2 hundreds and 10 tens, then 1 ten is decomposed into 10 ones (two steps); or 300 can be renamed directly as 2 hundreds, 9 tens, and 10 ones (one step). In each case, students use math drawings to model the decompositions and relate them to the written vertical form, step by step. In Lesson 18, students work with three-digit subtraction problems, which they apply multiple strategies to solve. For example, with 300 – 247, students learn they can use compensation to subtract 1 from each number, making the equivalent expression 299 – 246, which requires no renaming. They may also use the related addition sentence, 247 + ___ = 300, and then use arrow notation to solve, counting up 3 to 250 and then adding on 50, to find the answer of 53. For some problems, such as 507 – 359, students may choose to draw a chip model and relate it to the algorithm, renaming 507 as 4 hundreds, 9 tens, 17 ones in one step. As students apply alternate methods, the emphasis is placed on students explaining and critiquing various strategies.

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Common Core Learning Standards

CCLS State Standard
2.NBT.7 Add and subtract within 1000, using concrete models or drawings and strategies based on place value...
2.NBT.9 Explain why addition and subtraction strategies work, using place value and the properties of...

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