Let's assume that there are X students who initially signed up for the activity. If the number of students can be doubled, you can enjoy the discount. If they can enjoy the preferential treatment, then the average cost shared by each student participating in the activity is less than that of the original 4 yuan, which can be solved by an equation.

Answer: Answer: Suppose there are X students who originally reported to the class. According to the meaning of the question, they get

320 /X -480/2X=4

Solve this equation and get x = 20.

After investigation, x=20 is the solution of the original equation, which accords with the meaning of the question. A: There were 20 students who initially signed up.