Answer:
To determine the explicit rule for the arithmetic sequence of 20.5, 16, 11.5, 7, 2.5, ..., the following logical reasoning must be performed:
20.5 - 16 = 4.5
16 - 11.5 = 4.5
11.5 - 7 = 4.5
7 - 2.5 = 4.5
Thus, the explicit rule of the arithmetic sequence is that the numbers are gradually subtracted by 4.5, with which the next number of said sequence would be -2.