13. The height of sea water, \(h\) metres, on a harbour wall at time \(t\) hours after midnight is given by
$$h = 3.7 + 2.5 \cos ( 30 t - 40 ) ^ { \circ } , \quad 0 \leqslant t < 24$$
- Calculate the maximum value of \(h\) and the exact time of day when this maximum first occurs.
Fishing boats cannot enter the harbour if \(h\) is less than 3
- Find the times during the morning between which fishing boats cannot enter the harbour.
Give these times to the nearest minute.
(Solutions based entirely on graphical or numerical methods are not acceptable.)