I thought I had answered this before! First, you have "a1" and "d" swapped. "the last place team is awarded $ 275 in prize money" means that a1 is 275, not d. Second, you are asserting that the 16th position, the winning award, is 8000, not the sum.

The nth value is, then, and the sum, through n values is .

Since the sum of all 16 prizes is to be 8000, you need to solve for d and then find .