Ann Arbor, Michigan:

As of the 2010 U.S. Census, there were 113,394 people, 45,634 households, and 21,704 families residing in the city. The population density was 4,270.33 people per square mile (2653.47/km²). There were 49,982 housing units at an average density of 1,748.0 per square mile (675.0/km²), making it less densely populated than inner-ring Detroit suburbs like Oak Park and Ferndale (and than Detroit proper), but more densely populated than outer-ring suburbs like Livonia or Troy. The racial makeup of the city was 73.0% White (70.4% non-Hispanic White), 7.7% Black or African American, 0.3% Native American, 14.4% Asian, 0.0% Pacific Islander, 1.0% from other races, and 3.6% from two or more races. Hispanic or Latino residents of any race were 4.1% of the population.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". During what census were there 21,407 families residing in the city?
unanswerable