Hard
_aa_ba_bb_ab_aab (find the correct letters to fill the blanks)
Explanation
Explanation
The complete series follows a repeating 4-letter cycle: b,a,a,b (i.e., 'baab' repeated). Filling in the blank positions according to this cycle gives 'bbaab'.
PRO TIP
Re-check your working by substituting the answer back into the original question.