Let's see, we want the average increase of % in female students
First of things first, we need to know how much it increased year by year
So
1980 it was 6,7% and 1990 it was 10,2%
10,2% - 6,7% = 3,5%
The % increased in 3,5% from 1980 to 1990
Applying it to the others we have:
1980 to 1990 = 3,5%
1990 to 2000 = 3,1%
2000 to 2012 = 8,2%
So we have 3 percentages and we want the average, so lets sum them and divide by 3
(3,5 + 3,1 + 8,2)/3 = 14,8/3 = 4,933333...%
Rouding 4,93333...% we have 4,9%
