8. A batch of disaster relief supplies is loaded onto 26 trucks, which travel at a constant speed of $v$ kilometers per hour from a city to the disaster area. If the highway between the two places is 400 kilometers long, and the distance between every two trucks must not be less than $\left(\frac{v}{20}\right)^{2}$ kilometers, then how many hours at least are needed for all the supplies to reach the disaster area?
8. Let the total time required for all supplies to reach the disaster area be $t$ hours. The first truck takes $\frac{400}{v}$ hours to reach the disaster area, and the distance between the 26th truck and the 1st truck is at least $25 \times\left(\frac{v}{20}\right)^{2}$ kilometers. Therefore,
$t \geqslant \frac{400}{v}+\frac{25 \times\left(\frac{v}{20}\right)^{2}}{v}=\frac{400}{v}+\frac{v}{16} \geqslant 2 \sqrt{25}=10$. The equality holds if and only if $v=80$ kilometers/hour, and the distance between each pair of trucks is $\left(\frac{v}{20}\right)^{2}$ kilometers. In this case, $t=10$ hours.