Which set of letters, when placed in order in the blanks of the given letter series, will complete it?
a b _ c a _ _ a c _ b c _ a b b a
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → cbbac
Filled in, the series is made of the block of nine letters a b c c a b b a c, repeated; the second copy stops after eight letters.
Placing c, b, b, a, c in the blanks gives: abccabbac abccabba
Read in threes this is abc, cab, bac, then abc, cab, ba again
The given letters fix the blanks: the 10th place repeats the 1st letter (a), the 3rd place repeats the 12th (c), the 13th repeats the 4th (c), and the 6th and 7th repeat the 15th and 16th (b, b)
Why the other options are incorrect
Option (2) bcbac — Incorrect. It gives abbcacbac abccabba; the first nine letters do not repeat in the next eight.
Option (3) cabac — Incorrect. It gives abccaabac abccabba; the 6th letter a does not match the 15th letter b.
Option (4) bbcab — Incorrect. It gives abbcabcac abcbabba; neither part repeats the other.