18. Yangyang sets off from home to school. If she walks at 60 meters per minute, she arrives at 6:53. If she walks at 75 meters per minute, she arrives at 6:45. The time Yangyang sets off from home is $\qquad$ .
【Analysis】6:53 - 6:45 = 8 minutes. Let's assume that if Yangyang walks 60 meters per minute, it takes $x$ minutes to get to school, then if he walks 75 meters per minute, it takes $x-8$ minutes to get to school. Since the distance from home to school is constant, we can set up an equation based on "speed $\times$ time = distance" to solve the problem.

【Solution】Solution: Let's assume that if Yangyang walks 60 meters per minute, it takes $x$ minutes to get to school,
6:53 - 6:45 = 8 minutes
\[
\begin{aligned}
60 x & =(x-8) \times 75 \\
60 x & =75 x-600 \\
15 x & =600 \\
x & =40 ;
\end{aligned}
\]
6:53 - 40 minutes = 6:13;
Answer: Yangyang left home at 6:13.
Therefore, the answer is: 6:13.
【Comment】This question examines solving application problems by setting up equations. The key to solving this problem is to find the basic quantitative relationship according to the problem, set the unknown as $x$, and then set up an equation to solve the problem.