let 'n' represent the number, then we have two choices since "the difference of a number and 8" can be read as either "n-8" or "8-n":

one possibility:

2(n-8) = n-1/2

solving for 'n' we have n=15.5

the other possibility:

2(8-n) = n-1/2

solving for 'n' we have n=5.5

so n is probably 15.5 but could also be 5.5, the wording of the question is simply ambiguous