A hint to get started
Classify code words by the number of repeated pairs
Using the letters of the word CORRELATION, find the number of
(i) arrangements such that the two letters A and T are next to each other,
(ii) 6-letter code words that can be formed.
[2011/AJC/II/6(modified)]
CJC · Discussion · Question 10
Only O and R can repeat; classify the six-letter strings by zero, one or two pairs.