Task: Question Answering
Input (English): Passage: Lower than expected population figures from the census suggest that Karachis poor infrastructure, law and order situation, and weakened economy relative to other parts of Pakistan made the city less attractive to in-migration than previously thought. The figure is disputed by all the major political parties in Sindh. Karachis population grew by 59.8% since the 1998 census to 14.9 million, while Lahore city grew 75.3% – though Karachis census district had not been altered by the provincial government since 1998, while Lahores had been expanded by Punjabs government, leading to some of Karachis growth to have occurred outside the citys census boundaries. Karachis population had grown at a rate of 3.49% between the 1981 and 1998 census, leading many analysts to estimate Karachis 2017 population to be approximately 18 million by extrapolating a continued annual growth rate of 3.49%. Some had expected that the citys population to be between 22 and 30 million, which would require an annual growth rate accelerating to between 4.6% and 6.33%. 
Question: How many years was the time span where Karachis populations has grown at a rate of 3.49%? 
Answer type: Number 
Output (English): 17.