Grade: 3, Topic(s): addition/subtraction

## Purpose

To subtract 12 from any number using mental computation.

None

Choose a starting number (such as 100) and ask a student to subtract 12 from the number. Then ask the next student to subtract 12 from *that* result.

If children have difficulty, remind them of the strategy they have used to add 12: adding 10, and then adding 2 more. Similarly, to subtract 12, they can subtract 10, and then subtract 2 more.

Part 1 starts at a familiar number (100), allowing children to more easily subtract 10 (from a multiple of 10) and then count backwards 2 more. Part 2 begins at number that is not a multiple of 10 (i.e., 151). The extension uses another three-digit number that is not a multiple of 10.

Let’s start at 100 and subtract 12. What number do we have? (88). Now, let’s subtract 12 again from *that* number…and then again from *that* number until we get close to 0.

- 100, 88, 76, 64, 52, 40, 28, 16, 4

*If children need more practice or are enjoying their mastery, repeat. If children are ready to move on, go to Part 2.*

Let’s start at 151 and subtract 12. What number do we have? Now, let’s subtract 12 again from *that* number… and then again from *that* number until we get close to 0.

- 151, 139, 127, 115, 103, 91, 79, 67, 55, 43, 31, 19, 7

*If children need more practice, repeat. Or, when children seem excited for a new challenge, move to the extension.*

Let’s start at 199 and subtract 12. What number do we have? Now, let’s subtract 12 again from *that* number…and then again from *that* number until we get close to 0.

- 199, 187, 175, 163, 151, 139, 127, 115, 103, 91, 79, 67, 55, 43, 31, 19, 7

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