Answer:
85 is the correct answer, sorry I have to type 20 characters
85
From 121 to 109, it subtracted 12.
From 109 to 99, it subtracted 10.
From 99 to 91, it subtracted 8.
As you can see, the pattern is subtracting two less than the time before.
So for the next number you would do: 91-6= 85
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