|Please refrain from posting in all caps.|
All caps is the Internet equivalent of shouting and no one likes to be yelled at.
It might help if you provided an example of your data, with inputs and outputs, but I'll give it a shot:
Let's say you have a list of numbers in A1:A5:
In A6 you have =SUM(A1:A5)
In A7 use:
=IF(A6>150, A6 - A6*0.05, A6)
If A6 is greater than 150, return A6 minus 5% of A6. If not, return A6.
I hope that helps.