The parking fee is less than or equal to two hours. 5 yuan's mother finally paid 30 yuan. How long did she stop?

Take Xining as an example: The Maximum Parking Charge Standard for Residential Areas in Xining stipulates that the charging standard for parking spaces in residential areas is: the maximum price for above-ground parking spaces is 70 yuan/month, and the maximum price for underground parking spaces is 200 yuan/month; The maximum price of service charge for above-ground parking spaces is 30 yuan/month, and the maximum price of service charge for unsold underground parking spaces is 60 yuan/month. The temporary parking time of vehicles does not exceed 30 minutes (including 30 minutes), and the parking fee is free; Temporary parking in 5 yuan within 2 hours (including 2 hours), 2 hours to 8 hours (including 8 hours) 15 yuan, 8 hours to 24 hours (including 24 hours) 15 yuan. The property management company can appropriately reduce the charging standard within 20% according to the facilities and equipment configuration, ground material, service standard and parking space surplus of the underground parking lot.