There were 169,710 housing units at an average density of , of which 153,791 were occupied, of which 63,142 (41.1%) were owner-occupied, and 90,649 (58.9%) were occupied by renters. The homeowner vacancy rate was 3.0%; the rental vacancy rate was 8.5%.  166,662 people (42.7% of the population) lived in owner-occupied housing units and 215,924 people (55.3%) lived in rental housing units.. How many of the housing units were unoccupied?