In a block of flats, there are 120 apartments. 85% of them have at least 1 resident, while 60% of the apartments have at least two residents. How many apartments have only one resident? Let's answer step by step:
At least one person lives in 85 / 100 * 120 = 102 apartments. At least two persons live in 60 / 100 * 120 = 72 apartments. So in every of the 102 - 72 = 30 apartments, only one person is living. The answer: 30.