question: Richard lives in an apartment building with 15 floors. Each floor contains 8 units, and 3/4 of the building is occupied. What's the total number of unoccupied units In the building?
answer: The total number of units in the building will be 8 units/floor * 15 floors = <<8*15=120>>120 units.
  If 3/4 of the building is occupied, then the total number of occupied units is 3/4 * 120 units = <<3/4*120=90>>90 units.
  The total number of unoccupied units is 120 units - 90 units = <<120-90=30>>30 units.
  #### 30