Hard
a_ba_b_b_a_b (find the correct letters to fill the blanks)
Explanation
Explanation
The complete series follows a repeating 3-letter cycle: a,b,b (i.e., 'abb' repeated). Filling in the blank positions according to this cycle gives 'bbabb'.
PRO TIP
Re-check your working by substituting the answer back into the original question.