I:boatspeed-currentspeed=140
II:boatspeed+currentspeed=260
add both equations:
boatspeed-currentspeed+boatspeed+currentspeed=140+260
2*boatspeed=400
boatspeed=200
calculate currentspeed: insert boatspeed=200 into I:
200-currentspeed=140
60=currentspeed
those distance values were measured over 5 hours
so your boatspeed[km/h]=200/5=40[km/h]
and your current[km/h]=60/5=12[km/h]
