Hi AC, thanks for answering the question, but shouldn’t the answer be 40? I am confused as the answer sheet also state 39.
Example to find out which figure has 18 total Squares, we use
18-9 = 9
9/3 = 3
Then it should be 3+1= 4.
The figure number is 4, according to the question table given.

Hi Jackson R,
Your formula is correct. You can use -9 then divide 3 and +1 for Figure number.
A mistake during your calculations.
123-9= 114
114/3=38
Figure number = 38+1 = 39.
Hope this clear your doubts.

Thank you !