f(n) = -1.2n + 38.1
Hence, for an additional race
the finishing time, f(n+1), is given
f(n+1) = -1.2(n+1) + 38.1 = -1.2n + 38.1 -1.2
Which implies that
f(n+1) = f(n) - 1.2
This implies that the finishing time decreases by 1.2minutes
The model predicts that for each additional race a runner has run, the finishing time decreases by about 1.2 minutes