While the Earth spins with a 24-hour period, the Moon revolves around the Earth. Thus, it does not appear overhead to observers every 24 hours, but instead, about every 24 hours and 50 minutes. So the entire cycle, containing 2 high tides and 2 low tides, takes about 24 hours and 50 minutes, meaning high tides are separated by roughly half that time interval.
Work Step by Step
This is discussed on page 168.