The answer is 65.35
80 | 100
52 | 65.35
80/52 = 1.53
100/1.53 = 65.35
The answer is 0.6616
Hope it helped :)
Based on these first few terms, we can deduce that the next term is computed by switching the sign of the previous one, and multiplying it by 3: we start with -1, we switch the sign (1) and multiply by 3 (3); then again we switch the sign (-3) and multiply by 3 (-9), and so on.
Since switching sign is the same as multiplying by -1, we can compute every next term by multiplying the previous one by -3:

So, the recursive formula is

because it states precisely that the next term is -3 times the previous one.
The minimum thickness of the book would simply be the sum of the thickness of the two cover pages and all the pages in between. That is:
minimum thickness = 0.9 mm * 2 + 0.13 mm * 400
minimum thickness = 53.8 mm
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