2, enough to subtract, go to 5, complement.

3. The file is not enough 10.

4. If the number of files is not enough, subtract 10 and add the complement to make up the number: the two numbers add up to 5; Complement: The sum of two numbers is equal to 10.

5, nursery rhymes:

Add one, please dial double nine, we are all good friends; Add two, please dial double eight, laugh when you are happy; Add three, please dial double seven, sometimes you will be angry; Add four, please dial double six and jump if you are happy; If you subtract one, please dial double nine and sing like a lark.

If you subtract two, please dial double eight, don't delay too much; If you subtract three, please dial double seven and queue up neatly to do exercises; If you subtract four, please dial double six. Don't sleep late in the morning.

Abacus calculation is a digital calculation method with abacus as a tool. It is the national intangible cultural heritage of China and is known as the fifth invention of China. Abacus has corresponding rules for four operations, which are collectively called abacus rules. The abacus calculation uses abacus calculation formula to control the abacus calculation process, and various abacus calculations have different fingering and formulas.

Advantages: abacus can change learning attitude and increase children's concentration. Learning abacus mental arithmetic often requires independent and focused thinking, which invisibly cultivates children's attention and memory.

Disadvantages: abacus calculation conflicts with primary school mathematics, and it is easy to be confused without practice. Mathematics in primary school is from low to high, while mental arithmetic in abacus is just the opposite of primary school teaching. First high, then low. This opposite calculation method may have an impact on children's learning.