Subtracting 12 in Chains – Elementary Math

Purpose

To subtract 12 from any number using mental computation.

Materials

None

Overview

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.

About the sequence

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.

Part 1

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.

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.

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