13.197. A cinema hall has two doors, a wide one and a narrow one. After a screening, the audience exits the hall through both doors in 3 minutes and 45 seconds. If the audience is let out through only the wide door, it takes 4 minutes less than if they are let out through only the narrow door. How much time is required for the audience to exit the cinema hall through each door separately?
## Solution.

Let $x$ min be necessary to release the audience only through the wide door, $(x+4)$ min - only through the narrow door. In one minute, $\frac{1}{x}$ people exit through the wide door, and $\frac{1}{x+4}$ people exit through the narrow door. According to the condition: $\frac{1}{x}+\frac{1}{x+4}=\frac{1}{3 \frac{3}{4}}$, from which $x=6$ min.

Answer: 6 and 10 min.