If a fishing boat travels 15mph in still waters how long will it take it to travel 30 miles upstream with a current of 3?

The boat travels past the water around it at 15 mph.

If the water is moving past the riverbank at 3 mph, then the boat is moving past the riverbank

at (15 - 3) = 12 mph.

At 12 mph, it takes (30/12) = 2-1/2 hours (2hr 30min) to travel 30 miles up the riverbank.

Coming back, the boat's speed past the riverbank will be (15+3) = 18 mph.

It will take (30/18) = 1-2/3 hours (1hr 40min) to travel 30 miles down the riverbank.