Two friends paddle 3 hours upstream, then 4 hours downstream to camp, in a 3 mph current. Next day they reach the start, 48 miles upstream of camp, at 9:30. When did they leave?

Two friends canoe upstream for 3 hours, then realise the campsite is downstream and paddle downstream for 4 hours to reach it. The next day they paddle back to where they first set out, 48 miles upstream of the campsite, and arrive at 9:30. The current is a constant 3 mph and they always paddle at the same constant rate. What time did they leave? Candidates report this question with different numbers from one sitting to the next.