Task: Question Answering
Input (English): Passage: Ashkelon was formally granted to Israel in the 1949 Armistice Agreements. Re-population of the recently vacated Arab dwellings by Jews had been official policy since at least December 1948, but the process began slowly.  The Israeli national plan of June 1949 designated al-Majdal as the site for a regional Urban area of 20,000 people. From July 1949, new immigrants and demobilization soldiers moved to the new town, increasing the Jewish population to 2,500 within six months. These early immigrants were mostly from Yemen, North Africa, and Europe. During 1949, the town was renamed Migdal Gaza, and then Migdal Gad. Soon afterwards it became Migdal Ashkelon. The city began to expand as the population grew. In 1951, the neighborhood of Afridar was established for Jewish immigrants from South Africa, and in 1953 it was incorporated into the city. The current name Ashkelon was adopted and the town was granted Local council (Israel) status in 1953. In 1955, Ashkelon had more than 16,000 residents. By 1961, Ashkelon ranked 18th among Israeli urban centers with a population of 24,000. This grew to 43,000 in 1972 and 53,000 in 1983. In 2005, the population was more than 106,000. 
Question: By how many people did the population of Ashkelon increase from 1955 to 1961? 
Answer type: Number 
Output (English): 8000.