Mixed Costs and Cost Formula

Callie's Gym is a complete fitness center. Owner Callie Ducain employs various fitness trainers who are expected to staff the front desk and to teach fitness classes. While on the front desk, trainers answer the phone, handle walk$-$ins and show them around the gym, answer member questions about the weight machines, and do light cleaning $($wiping down the equipment, vacuuming the floor$).$ The trainers also teach fitness classes $($e$.$g$.,$ pilates, spinning, body pump$)$ according to their own interest and training level. The cost of the fitness trainers is $$600$ per month and $$20$ per class taught. Last month, $100$ classes were taught.