Per capita ship 20÷4=5 (yuan).
A big ship is cheaper than a small one. If a small boat can't be full, rent as many big boats as possible.
45-6×7=3 people, and the remaining 3 people rent a boat.
Rent 6 big boats, 1 small boat.
6×28+20= 188 (yuan)
This is that most economical method.
If two teachers also want to take a boat, then
47-5×7= 12 (person)
12÷4=3 (only)
Rent five big boats and three small boats.
5×28+3×20=200 (yuan)