There were 78,003 housing units at an average density of 2,446.5 per square mile (944.6/km²), of which 44,914 (60.5%) were owner-occupied, and 29,371 (39.5%) were occupied by renters. The homeowner vacancy rate was 1.1%; the rental vacancy rate was 5.4%. 115,470 people (60.8% of the population) lived in owner-occupied housing units and 73,632 people (38.8%) lived in rental housing units.. How many housing units were vacant?