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It's 18. Piece of cake

Peace of cake? 18 is wrong @rkdupron !

this question can have 18 as an answer as well.
11 x 11= 121 =>1+2+1=4
22 x 22= 484 =>4+8+4=16
33 x 33= 1089 =>1+0+8+9=18

Pls correct me if I'm wrong.

Seems right.
This is my calculation.
11×11=1+1×1+1=(2×2) 4
22+22=2+2×2+2=(4×4) 8
33×33=3+3×3+3=(6×6) 36

Sure, but pls do think about the other answers that r possible as well before posting these questions.
They are great, but different people see them in different angles.
If possible, maybe u can give a bit more hints that lead towards the solution that you got.

P.S. I upvoted ur comment.

Thank you.
Youre completely right.