Grades 2–3
Add with a Compensation Strategy
Add the next ten, then give back the extra 1 you put in.
19, 29, and 39 are awkward because they sit one below a ten. Adding the ten is easy: 27 + 20 lands on a number you can see. You put in 1 too many, so you give that 1 back at the end. The total does not change if the extra goes in and then comes back out.
27 + 19. 19 is 1 less than 20. First add the 20: 27 + 20 = 47. Then take the extra 1 away: 47 − 1 = 46. So 27 + 19 = 46.
- See how close the second number is to the next ten.
- Add that ten instead.
- Subtract the extra you put in.
- The amount you subtract is the same amount you added on.
36 + 29 uses the same move. 29 is 1 less than 30. 36 + 30 = 66, then 66 − 1 = 65. 48 + 19: 48 + 20 = 68, then 68 − 1 = 67.
Watch out
- Adding the 1 at the end instead of subtracting it. You already put that 1 in.
- Changing only one addend and leaving the total unfixed.
- Subtracting 10 when you only borrowed 1 to reach the ten.
What is 36 + 29?
36 + 30 = 66, then 66 − 1 = 65, because 29 is 1 less than 30.
What if the number is 2 below the ten, like 28?
Add the ten, then subtract 2. 35 + 28 is 35 + 30 = 65, then 65 − 2 = 63.
Math practice for Compensation Strategy
Add within 100 without regrouping
Practice this