Assignment task: Irene and Katie work at the same coffee shop. During a break, Katie, who has been on the job longer, finds out that Irene is earning the same hourly rate as she is. Katie trains new employees and has substituted for her shift supervisor when he calls in sick. She knows Irene is popular with customers and the staff, nevertheless, Katie is feeling as though she has been treated unfairly. What theory explains Katie's feelings? How can her manager address this situation?