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Ahh I see.

Time taken upstream is 5. Time taken downstream is 2.5

So speed of river downstream is twice as much as upstream.

Erm... what do you do next to be able to form the appropriate...

Sorry my mistake...

I don't understand what you did. Is that a standard rule? Or application of knowledge?

Speed = Distance / Time

part a) If it takes 5 hours at a speed of 3mph. Then the distance traveled is 15 miles
part b) If it takes 2.5 hours at a speed of 3mph. Then the distance traveled is...