7. There is a bus, a truck, and a car each traveling in the same direction at a constant speed on a straight road. At a certain moment, the bus is in front, the car is at the back, and the truck is exactly in the middle between the bus and the car. After $10 \mathrm{~min}$, the car catches up with the truck; after another $5 \mathrm{~min}$, the car catches up with the bus; then after $t \min$, the truck catches up with the bus. What is the value of $t$?
7. 15 .

Suppose at a certain moment, the distances between the truck and the bus, and the truck and the car are both $S \mathrm{~km}$. The speeds of the car, truck, and bus are $a, b, c(\mathrm{~km} / \mathrm{min})$, respectively, and it takes the truck $x \mathrm{~min}$ to catch up with the bus.
From the problem, we have
$$
\begin{array}{l}
10(a-b)=S, 15(a-c)=2 S, \\
x(b-c)=S .
\end{array}
$$

Thus, $30(b-c)=S$.
Therefore, $x=30$.
So, $t=30-10-5=15(\mathrm{~min})$.