by C. Elkins, OK Math and Reading Lady
This is part three in a series of strategies regarding addition and subtraction strategies. This part will focus on a variety of strategies to help toward memorization of facts, meaning automatic computation. While children are learning their number bonds (building up to 5 in KG, to 10 in first grade, and to 20 in second grade), there are other facts which cross several number bonds that students can work towards. These strategies to build mental math automaticity are highlighted below. Get some freebies in the section on doubles / near doubles.
Identity (or Zero) Property:
- The value of the number does not change when zero is added or subtracted.
- 3 + 0 = 3
- 9 – 0 = 9
- The answer is always zero when you take away / subtract all.
- 9 – 9 = 0
- 50 – 50 = 0
Adding 1 or Subtracting 1:
- Adding 1 results in the next number in the counting sequence.
- Subtracting 1 means naming the number that comes right before it in the counting sequence.
- With manipulatives, lay out an amount for student to count. Slide one more and see if he/she can name the amount without recounting.
- Do the same as above, but take one away from the group to see if he/she can name the amount without recounting.
- Show this concept using a number line.
- 6 + 1 = 7; 26 + 1 = 27
- 7 – 1 = 6; 37 – 1 = 36
- After +1 or -1 strategies are in place, then go for +2 or -2 for automatic processing.
Next-Door Neighbor Numbers:
- If subtracting two sequential numbers (ie 7 subtract 6), the answer is always one because you are taking away almost all of the original amount.
- Help students identify these types of problems: 8-7; 10-9; 98-97; 158-157
- Guide students to writing these types of problems.
- Relate these to subtracting 1 problems. If 10-1 = 9; then 10 – 9 = 1.
- Show on a number line.
Doubles (with freebies): Continue reading