Answer
They have traveled $16.4$ mi from the start.
Work Step by Step
$\bf{\text{l. Familiarize yourself with the problem.}}$
The time given here is not of importance.
Given a total distance 20.6 mi, we need to find the distance from the start, x, to satisfy the given condition.
$\bf{\text{2. Translate to mathematical language. (This often means writing an equation.)}}$
x is four times larger than (20.5-x), the distance to the finish.
$x=4(20.5-x)$
$\bf{\text{3. Carry out some mathematical manipulation. (This often means solving an equation.)}}$
... simplify the RHS
$ x=82.0-4x\qquad$... add 4x to both sides
$ 5x=82.0\qquad$... divide both sides with 5
$x=16.4$ mi
$\bf{\text{4. Check your possible answer in the original problem.}}$
If they are at $16.4$ mi from the start, they have another
$ 20.5-16.4=4.1\quad$ to the finish.
Is $16.4$ four times as much as $4.1$?
Yes.
$\bf{\text{5. State the answer clearly using a complete sentence.}}$
They have traveled $16.4$ mi from the start.