Which group should replace the question mark in the series K3M, N6P, Q9S, T12V, ?

The leading letters run K, N, Q, T — each three ahead of the last — so the next is W (23rd). The trailing letters run M, P, S, V in the same way, so the next is Y (25th). The numbers run 3, 6, 9, 12 and so give 15. The group is W15Y. W15X takes the trailing letter only two ahead of V; X15Y moves the leading letter four ahead of T; W18Y adds six to the number instead of three.