Assuming you had an equal chance at getting each horse then the probability of winning would be 1 in 17, regardless of how the odds on each horse were distributed. The player’s expected return can be expressed as (1/17)*15.3 + (16/17)*-1 = -0.0412. In other words the house edge is 4.12%. However there is another simple formula you can use. If a = actual odds paid and f = fair odds for bet then the house edge is (f-a)/(f+1). In this case the fair odds are 16 to 1. So the house edge is (16-15.3)/(16+1) = 0.7/17 = 4.12%.