Q: There are k animal feeding stations arranged in a line with a supply hut. The stations are s yards apart and the nearest is t yards from the hut. An attendant carries n bags of feed, one at a time, to each feeding station. How far will he have traveled when he arrives back at the hut after servicing all stations?

Here's what I did, but I don't know if it's correct: (S.k denotes 'S of k')

S.k=(2t + 2(t+s)+...+2(t+(k-1)s))n

= n((k/2)(2(2t)+(k-1)2s))

= n((k(4t+2sk-2s))/(2))

= n((4kt+2sk^2-2sk)/(2))

= n(kt+sk^2+sk)

= nk(t+sk+s) yards