Teresa has a nail polish selling business. She can sell her polish individually for $12 each, or in sets of 4, for $36. (They are “buy 3 get one free” when sold in sets.) She has set a sales goal for the month of $300 so that she can earn her team bonuses. First, set up an equation to represent the total number of individuals (i) and sets (s) she needs to sell to reach her goal.