1. The problem statement, all variables and given/known data Figure 2-21 shows a general situation in which a stream of people attempt to escape through an exit door that turns out to be locked. The people move toward the door at speed vs 3.50 m/s, are each d 0.25 m in depth, and are separated by L 1.75 m.The arrangement in Fig. 2-21 occurs at time t 0. (a) At what average rate does the layer of people at the door increase? (b) At what time does the layer’s depth reach 5.0 m? (The answers reveal how quickly such a situation becomes dangerous.) 2. Relevant equations 3. The attempt at a solution I am not certain how to solve this.