Subtraction Math Strategies
Subtraction Math Strategies
Zero Facts: There are two types in subtraction. One is, if there is a zero, the answer is always the other number (same as addition). Some kids need to cross the zero out to help them think through the process. The other is, if both numbers are the same, the answer is zero. If they tend to double it as an addition double problem, I ask them to put up however many fingers (7-7, they'd put up 7 fingers) and then ask them how many do they take away.
Subtraction Doubles: Test for this if the number sentence is not a zero fact. If there is a teen in the problem then it is either a subtraction double or a magic nine. Cover up the larger number. Double the lower number. Uncover the larger number. If the larger number, you just uncovered, is the answer to the double number sentence, it's a subtraction double. The answer is the number you doubled. Example: 18-9. Cover up the 18. Double the 9 by saying 9+9 equals 18. The larger, covered number is 18. It's a subtraction double and the answer is 9.
Count Backs: If there is a 1, 2, or a 3. You use the same touch point dots as in addition count on's. You start with the higher number, you point to it, and then point to the dots while counting backwards until you find your answer.
Magic Nines: There are 2 kinds of magic nines. Both kinds have to have a teen as one of the numbers you see in the number sentence. You say the following story (the reverse of the addition magic nine story): "The nine is tired of being a ten and it just wants to be a nine again. So the nine gives one back to his buddy (the other number) and add one to the number in the ones place of the teen number. Example: 17-9, so the 9 gives one back to the 7 and your answer is 8. The second magic nine still has a teen in it, but the answer is 9, it's not in the problem itself. You identify this one by covering up the 1 in the 10's Place of the teen number. If the numbers fall next door to each other on the number line the answer is 9. Example: 12-3. Cover up the one in the number 12. Two and three are next door to each other on the number line, so the answer is 9. Believe it or not, it works every time :)!
Count Up's: Numbers that are close together on the number line (or don't fall into any other strategy as this one always works). It's a "catch-all" strategy. Start at the lower number and shake your fist as you count. Count up to the higher number, putting up a finger for every number you count. When used correctly, answer is always a 1, 2, or a 3. If you use it as a "catch-all" the answer can be anything. Example: 6-4. Shake your fist as you say 4. Put up one finger as you say 5. Put up another finger as you say 6. You are at the highest number so you stop counting . You have 2 fingers up so the answer is 2.
