Using 10 As An Intermediate Stop
Introduction, Stage 4 – Mental Math – Single-Digit Add & Sub
For problems with numbers larger than 10, such as 13 – 8, break them into two differences. Make 10 an intermediate stop when going between the two numbers. The distance from 13 to 8 is the distance from 13 to 10 plus the distance from 10 to 8. Using this, 13 – 8 becomes (13 – 10) + (10 – 8) = 3 + 2 = 5.
This simplifies the mental load considerably by breaking these subtractions into two manageable parts. Subtracting 10 from a number between 10 and 20 is very straightforward. Learning how to subtract numbers from 10 is a matter of learning the number bonds for 10.