...luggage, stumble to their cars, and? and proceed to the parking lot attendant to pay for their parking. Traveler interarrival times are exponentially distributed, as are the service times of the attendant. On average, travelers arrive every 25 seconds. The attendant can process three travelers per minute, and processing rates follow a Poisson distribution.

How many minutes per hour is the attendant not serving customers?

I found that the answer is fewer than or equal to 13 but I can't seem to understand how to get that answer. Does anyone know? Thanks