Exercise 1 solution

Interval = 12 h 25 min.
First high water: Monday 06:00.
Second: 06:00 + 12:25 = 18:25 Monday.
Third: 18:25 + 12:25 = 06:50 Tuesday.
Fourth: 06:50 + 12:25 = 19:15 Tuesday.

So the three following high waters are about 18:25 Monday, 06:50 Tuesday and 19:15 Tuesday, and the Tuesday morning high water is about 06:50, 50 minutes later than on Monday. (Real tide tables differ by a few minutes to tens of minutes, because the interval varies through the month and with local conditions.)