60/3 = 20 mph with
60/5 = 12 mph against
The speed of the current adds to or subtracts from the speed of the boat in still water.
b + c = 20
b - c = 12
2b = 32
b = 16
16 + c = 20
c = 4
The cabin cruiser travels 16 mph in still water, and the current is 4 mph.