4. A rescue boat, whose maximum speed is \(20 \mathrm {~km} \mathrm {~h} ^ { - 1 }\), receives a signal which indicates that a yacht is in distress near a fixed point \(P\). The rescue boat is 15 km south-west of \(P\). There is a constant current of \(5 \mathrm {~km} \mathrm {~h} ^ { - 1 }\) flowing uniformly from west to east. The rescue boat sets the course needed to get to \(P\) as quickly as possible. Find

1. the course the rescue boat sets,
2. the time, to the nearest minute, to get to \(P\). When the rescue boat arrives at \(P\), the yacht is just visible 4 km due north of \(P\) and is drifting with the current. Find
3. the course that the rescue boat should set to get to the yacht as quickly as possible,
4. the time taken by the rescue boat to reach the yacht from \(P\).