We have to calculate the average increase in the percentage, and the extrapolate it to 2025.
We have the following data:
1980: 6.5%
1990: 10.6%
2000: 13.3%
2012: 21.1%
We can calculate an average value taking into account the year 2012 and the year 1980.
We can do that as:
In 32 years, the average increase in the percentage is 0.45625%.
Then, we have to extrapolate to the year 2025.
From 2012 we have (2025-2012) = 13 years.
Then, we expect an increase of:
From 2012 to 2025 we expect an increase of 5.9%, so we add it to the level of 2012 and we get the level of 2025:
The percentage of first year college females who will claim no religious affiliation in 2025 is approximately 27.0%.