Answer
0.21 h, or about 13 minutes.
Work Step by Step
The drop in sea level means that a trough has just arrived (see figure 11–24). The peak of the wave is just one-half wavelength behind the trough, and the wave is traveling at 550 km/h.
The arrival time is the distance divided by the speed.
$$t=\frac{\lambda/2}{v}=\frac{235km}{2 \times 550 km/h}\approx 0.21h$$
This is about 13 minutes. Run!
