Given X is assumed to be the total number of friends, in 8(X-1) + 4, you need to minus 1 from X because the last friend only received 4 stickers, hence Jane had to top up 4 stickers for the last friend. Another way of looking at it is, only the first (X-1) friends received 8 stickers each, and the last friend received the remaining 4 stickers.