To find the number of occupied units in the apartment building, first, we need to determine how many units are vacant.
Given that 15% of the units are vacant, we can calculate the number of vacant units as follows:
\[ \text{Vacant units} = \text{Total units} \times \text{Percentage of vacant units} \] \[ \text{Vacant units} = 120 \times 0.15 = 18 \]
Now, to find the number of occupied units, we subtract the number of vacant units from the total number of units:
\[ \text{Occupied units} = \text{Total units} - \text{Vacant units} \] \[ \text{Occupied units} = 120 - 18 = 102 \]
Therefore, the number of occupied units is 102.