There is a way to turn this into a simple equation, but I will not go through that.
So the conditions are saying the first term is 40 and for every term after that you multiply the last by .5. So to get the fifth you would multiply the fourth by .5.
We want the third, so we need to multiply the second by .5. To get the second we multiply the first by .5. since the first is 40, 40*.5=20. Then just repeat.
