Can somebody help me with this question,
From a piece of wire 5000mm long, n pieces are cut,each piece 10mm longer
than the previous piece. If u is the lenght of the i-th piece and Si is the
cumulative lenght of i pieces.

(i) Express Un and Sn in terms of Ui and n
(ii)Determine the number of pieces that can be cut if Ui is 155mm

Could you please explain the answer

thanks