Jock decided to swim 10 km up Bega River. He swam 2 km every hour, then floated on his back for 10 minutes. Each time he floated, the current dragged him back 1 km. How long did it take Jock to complete the swim?

Effectively Jock gets 1km further every round of 1:10 hrs. After 8 rounds he has reached 8km (after just having floated back from 9 to 8). The next round he completes the journey without floating.

So total 9 hours of swim and 8*10 minutes of float, that's 10 hours and 20 minutes.