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?
At least one person lives in 85/100 * 120 = 
<gadget id="calculator">85/100*120</gadget>
<output>102</output>
102 apartments.
At least two persons live in 60/100 * 120 = 
<gadget id="calculator">60/100*120</gadget>
<output>72</output>
72 apartments.
So in every of the 102 - 72 = 
<gadget id="calculator">30*1</gadget>
<output>30</output>
30 apartments, only one person is living.

<result>30</result>