When doing the problem. If we use equations, we can grasp the characteristics of the constant total number of people to make equations. If you use a formula, you can grasp the characteristics of the constant total amount of money to make a formula.

Equation: Solution: A set of X elements, then the equation can be listed as follows:

30x+840=35x+ 120。 Move the project to get:

840=5x+ 120。 If you move the item again, you will get:

720=5x。 If the coefficient becomes 1, we get:

144=x

A: A sportswear 144 yuan.

Formula: (840-120) ÷ (35-30) =144 (yuan).

A: A sportswear 144 yuan.

It should be like this. I hope my answer can bring you some help. I wish you progress in your study! !