I have an finite artihmetic series of 600, 700, 800, 900, 1000, 1100. I am trying to create an equation to calculate the sum of the last x terms of the series. I think my problem is the fact that the length of the series changes based on what element I am at. If I want to know what element 4 5 and 6 equal it is not the same as just calculating the sum of the series at element 6.

Thanks in advance for any help.