A boat travels210 miles downstream and back. The trip downstream took 10 hours. The trip back took 70 hours. What is the speed of the boat in still water? What is the speed of the current?

1 Answer
Gió
Oct 3, 2015

I considered that the trip downstream is 210"miles" AND the trip upstream is 210"miles" as well but if it is different simply substitute into the explanation.

Explanation:

Call the speed of the boat v_b and the speed of the current v_c so you have:
downstream: v_b+v_c=210/10=21
upstram: v_b-v_c=210/70=3
where I used the fact that speed=distance/time;
from the second equation we have:
v_b=3+v_c substitute into the first equation:
3+v_c+v_c=21
2v_c=18
v_c=18/2=9"miles"/(hr)
and in v_b=3+v_c gives you:
v_b=3+9=12"miles"/(hr)

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