The depth of the water at the end of a pier changes periodically along with the movement of tides. On a
particular day, low tides occur at 12:00 am and 12:30 pm, with a depth of 2.5 m, while high tides occur at
6:15 am and 6:45 pm, with a depth of 5.5 m. Let t = 0 be 12:00 am.
How many times during this day is the depth at the end of the pier equal to 4 meters?