If we are to set up an equation to determine the number of movie tickets Erica purchased, we need to use variables to do that. Let's let movie tickets be t, and meals be m. If each movie ticket earns her $3, we will represent that as 3t; if each meal earns her .80, we will represent that as .8m. We are told next that she pruchased 15 meals and some unknown number of movie tickets and she earned $18 from it. We know that each meal earns her ,80, so in 15 meals she earned 15*.8 which is $12. The equation then will look like this: 3t+12=15. That's for Part A. That's the equation. Solving it for part B...we will subtract 12 from both sides to get 3t = 6, and divide both sides by 3. That's part B. Part C has us solving for t. Dividing both sides by 3 gives us that t = 2. That's the number of movie tickets Erica purchased last month.