hop question

im going to be brewing a long trail ale clone ina few days and im confused by the directions in regards to the hops.

The recipe says to first put in 4 aau's tettnang to boil
after 30 min: add 2 aau tettnang
after 30 more min ad 2 aau tettnang

I have two pacakges of 1 ounce of 3.7alpha tettnang pellets

For some reason i rember something like %alpha times 99 times ounce but cant remeber how do i diveide these packets up for the recipee i figured it means to add one package at the begining and then half of the other one and then the other half but not completly sure im sure this is an easy assumption but just curious this is my fourth beer and what will seem to be the most difficult gotta rembers to rdwhahb.

4 aau = 1 oz. @ 4 AA%, which is really close to 3.7%. One oz. at the beginning, 1/2 oz after 30 minutes, 1/2 oz. after 60.

thanks i figured it was that simple. But because im new to this what is the purpose off adding the same hops at different parts of the boil. I imagine to realease different flavors/ aspects of the hops.