Recall your d = rt, distance = rate * time.
bearing in mind that, when she's going downstream, she's not going 40 mph, she's going 40 + 4 mph, because the stream is adding its speed to it.
likewise, when she's going upstream, she's going 40 - 4 mph, because the stream is eroding speed from it, thus
so, "u" is 16 minutes and 40 seconds.
and "d" is 13 minutes and about 38 seconds.
bearing in mind that, when she's going downstream, she's not going 40 mph, she's going 40 + 4 mph, because the stream is adding its speed to it.
likewise, when she's going upstream, she's going 40 - 4 mph, because the stream is eroding speed from it, thus
so, "u" is 16 minutes and 40 seconds.
and "d" is 13 minutes and about 38 seconds.
