If the student runs 1 kilometer in 4 minutes,
then in 1 minute, he will run 1/4 kilometer.
Thus in x minutes, he will run x * 1/4 kilometers.
He starts out with 10 kilometers ahead of him, but every x minutes, he will run x/4 kilometers and so the 10 kilometers will get less and less.
f(x) = 10 - x/4
where 0 <= x <=40
