Let s and d represent the number of sit-ups Jerry does and the number of days Jerry has been doing them (not counting the first), respectively. The problem statement tells us
 s = 16 + 2d
This is the equation that can be solved to find d.
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Here is its solution.
Since we want to find d (the number of days), we can subtract 16 and divide by 2.
 s - 16 = 2d
 (s - 16)/2 = d
 d = (s/2) - 8
Example:
 Today Jerry did 22 sit-ups. He has been doing them for ...
 (22/2) - 8 = 11 - 8 = 3 . . . days
