In the group of letters PLUM the first and the third letters are each replaced by the letter two places ahead of them in the English alphabet, while the second and the fourth are each replaced by the letter two places behind them. What is the resulting group of letters?

P at place 16 goes forward two to R; L at 12 goes back two to J; U at 21 goes forward two to W; M at 13 goes back two to K. The group is RJWK. Moving every letter forward two gives RNWO; moving every letter back two gives NJSK; treating the last letter as one to be moved forward, and so forgetting that the fourth goes back, gives RJWO.