Step 1: In the initial stage of learning to calculate, children will put objects together and break their fingers. After mastering the movements, they can quickly put out the process of addition and subtraction.

Step 2 Image: In the process of children's learning, try to guide children to add and subtract in order. For example, fingers can be added and subtracted in sequence, which can be 4+5=9. How about 5+4? Ask your child why, guide him to think backwards and practice repeatedly, and he will feel that mathematics is diverse and not boring.

Step 3: Abstraction: In the process of sequential thinking, play with children in different ways gradually, and solve problems without fingers and eyes, so that children can have a process from image to abstraction.

y？ =4*x，y？ =0.85*x+ 15

2.4x=0.85x+ 15 Solve the equation x=4.7≈5, that is, it is more cost-effective to buy less than 5 from the supermarket and more than 5 from Taobao.