Also, I'd say that finding the Y intercept in this question is pretty strange, because in this case, X can only be a positive integer.

It would make sense for this problem that the Y intercept is when X = 0, because he's not running yet. But if you try to graph the following values, you'll see that the result is not a linear equation:

X | Y
-------
0 | 0
1 | 2
2 | 3.5
3 | 5
4 | 6.5
5 | 8