Lisa and daisy work at a hair salon. the salon charges $18 for a hair styling session with lisa and $12 for a session with daisy. the income on a certain day is projected to be $216. this situation can be represented by the equation 18x + 12y = 216, where x is the number of lisa's customers and y is the number of daisy's customers. how many customers would lisa need to serve to attain the projected income if daisy calls in sick that day?