Subtraction Compensation
Introduction, Stage 4 – Mental Math – Single-Digit Add & Sub
Compensation for subtraction means adding or subtracting the same amount to both num- bers. By adding or subtracting the same thing, the distance or difference between the two numbers is maintained.
Use compensation on 13 – 8 by adding 2 to both numbers to turn the problem into 15 – 10. Notice how much easier the problem became using this minor adjustment!
Single-digit problems can also be done this way. For example, 3 can be added to both numbers in 7 – 3 to make it 10 – 6, which is 4.