The speed, v, of the stream is found from the drifting canoe, which is 1/2 mile in 15 minutes, so v=2 miles per hour. When the canoe is paddled against the current at speed u, its net speed is u-2 mph. We know that 3(u-2)/4=2 so we can calculate the paddling speed: 3u-6=8, u=14/3 mph.
Therefore to paddle 8 miles against the current it will take time t where 8=(14/3-2)t.
24=(14-6)t, t=24/8=3 hours.
[Note that it wasn’t necessary to know the paddling speed because the canoe was only paddled upstream against the current. We could have simply used u-2=8/3, so 8=8t/3 and t=3. If the canoe had been paddled downstream, we would have needed the paddling speed.]