The very busy freshman class is selling tickets for an upcoming field hockey game. The class sells 20 more students tickets than adult tickets. The adult tickets cost $4 and the student tickets cost $2. The class made a total of $280 from ticket sales. How many of each type of ticket did they sell? Elaborate and clarify your reasoning.

1a. Define the variable in this system.
b. Create equations to describe the relationship in this system between the variables.

2. Solve the system of equations by using substitution method.