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I have tried to solve this sequence which, I suppose has been created from the combination of a,b,c,d,e. I've reposted the correct sequence
The sequence:
(a,c), ?, ?, ?, (a,e), ?, ?, (c,d), ?, (b,a)
My logic: We just try to do all the possible combinations :
(A,B) (A,C) (A,D) (A,E)
(B,A)...