What Is The Formula For Decrease By A Percentage Over Time?

It is very easy to do calculations for such percentage sums. Following the steps below:
Population decreased at the end of first year 7% of 20000 = 1400
Total Population at the end of first year = 20000-1400= 18600
Population decreased at the end of 2nd year 7% of 18600 = 1302
Total Population at the end of 2nd year = 18600-1302 = 17298
Population decreased at the end of 3rd year 7% of 17298= 1210.86
Total Population at the end of 3rd year = 17298-1210.86=16087.14