Answer
$20.83$ feet
Work Step by Step
Given $$s=\sqrt{30 f d},\tag{1}$$ where $s$ is the speed of the car in mile per hour, $f$ is the coefficient of friction on the road and $d$ is the length of the skid mark on the road.
Given that $s= 25$ mph and $f= 1$, we can find $d$ by solving equation $(1)$:
\begin{equation}
\begin{aligned}
s^2 & =\left( \sqrt{30\cdot 1\cdot d} \right)^2 \\
s^2& = 30d \\
\frac{s^2}{30}& =d\\
\frac{625}{30}&= d\\
d&= \frac{125}{6}\approx 20.83.
\end{aligned}
\end{equation}
