This task is about writing a correct answer for the reading comprehension task. Based on the information provided in a given passage, you should identify the shortest continuous text span from the passage that serves as an answer to the given question. Avoid answers that are incorrect or provides incomplete justification for the question.

Passage: From the 2011 Census, the Office for National Statistics published that Plymouth's unitary authority area population was 256,384; 15,664 more people than that of the last census from 2001, which indicated that Plymouth had a population of 240,720. The Plymouth urban area had a population of 260,203 in 2011 (the urban sprawl which extends outside the authority's boundaries). The city's average household size was 2.3 persons. At the time of the 2011 UK census, the ethnic composition of Plymouth's population was 96.2% White (of 92.9% was White British), with the largest minority ethnic group being Chinese at 0.5%. The white Irish ethnic group saw the largest decline in its share of the population since the 2001 Census (-24%), while the Other Asian and Black African had the largest increases (360% and 351% respectively). This excludes the two new ethnic groups added to the 2011 census of Gypsy or Irish Traveller and Arab. The population rose rapidly during the second half of the 19th century, but declined by over 1.6% from 1931 to 1951. Question: How many people did the Plymouth unitary authority gain between 2001 and 2011?
15,664