7. (5 points) Two mischievous children walk against the direction of the moving escalator, from one end to the other. The boy took 100 seconds, and the girl took 300 seconds. It is known that when the escalator is stationary, the boy walks 3 meters per second, and the girl walks 2 meters per second. Then the length of the escalator is $\qquad$ meters.
【Solution】: $(3-2) \div\left(\frac{1}{100}-\frac{1}{300}\right)$,
$$
\begin{array}{l}
=1 \div \frac{1}{150}, \\
=150 \text { (m). }
\end{array}
$$

Answer: The length of the escalator is 150 meters.
Therefore, the answer is: 150.