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? (Note: x ≥ 0, y ≥ 0, and x and y take only integer values.)