Question 1132693
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The formal algebraic solution by tutor @ikleyn is fine, and well presented.<br>
However, with the information given, this problem can be solved mentally with a bit of trial and error.<br>
Since the rate of the current is 4mph, the difference between the upstream and downstream speeds will be 8mph.<br>
Then, since the total time for the trip was EXACTLY 16 hours, the speed of the boat in still water will almost certainly be a whole number; and then the times for the upstream and downstream trips will be whole numbers.<br>
A very small bit of mental arithmetic finds that two speeds (mph) that differ by 8 and are factors of 48 (miles) are 4 and 12.<br>
And indeed these speeds work with the given information -- the total time for the trip is 48/4+48/12 = 12+4 = 16 hours.<br>
So the upstream and downstream rates are 4mph and 12mph; since the rate of the current is 4mph, the rate of the boat in still water is 8mph.