Question

Jens motorboat travels at a speed of 40 miles in Stillwater booth river flows at a speed of 4 mph how long will it take Jen to travel 10 miles upstream and 10 miles downstream

Answer 1

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


`\bf \begin{array}{lccclll} &\stackrel{miles}{distance}&\stackrel{mph}{rate}&\stackrel{hours}{time}\\ &------&------&------\\ Upstream&10&40-4&u\\ Downstream&10&40+4&d \end{array} \\\\\\ 10=(40-4)u\implies \cfrac{10}{40-4}=u\implies \cfrac{10}{36}=u\implies \cfrac{5}{18}=u \\\\\\ 10=(40+4)d\implies \cfrac{10}{40+4}=d\implies \cfrac{10}{44}=d\implies \cfrac{5}{22}=d`

so, "u" is 16 minutes and 40 seconds.

and "d" is 13 minutes and about 38 seconds.