The birth of a country in the years 1980 and 1990 are 8% and 9% respectively. find average birth?

You have not given enough information to indicate any particular answer.

Suppose one country has 100 population and 8 babies, while the other has 1000 population and 90 babies. The average birth rate is 98/1100 = 8.9%

Now suppose one country has 1000 population and 80 babies, while the other has 100 population and 9 babies. The average birth rate is 89/1100 = 8.1%

So you need to know "percent of what" for both countries to get any answer.