For example: 12-9 = 3

When thinking about solving a problem, first divide 12 into two numbers, an integer 10 and a 2. Because it is simple to subtract a number from an integer 10, it is easy to work out the answer. So 10 is in the box, and a triangle is drawn outside to point to 9, indicating 10-9. Now use 10-9 = 1, keep 1 in mind, and then use it to split the rest.

2 plus 1, the final answer is 3. See the figure below.