HKD1710 wrote:
In a given river, the current is 6 mph. A certain riverboat can travel 18 mph in still water. How far upstream (against the current) can the boat travel if a round trip is to take 10 hours?
A) 24
B) 48
C) 60
D) 80
E) 96
Another approach: set distances equal. The boat covers the same distance on both legs of the trip.
D = r*t for each leg of the trip (r = speed)
Upstream speed: (still water speed) - (speed of current)
Downstream speed: (still water speed) + (speed of current)
Total time is 10 hours.
If one leg's time =
t, other leg's = (10 - t)
Upstream legSpeed (rate) = (18-6) = 12
Time = t
Distance = r*t = 12t
Downstream legSpeed (rate) = (18+6) = 24
Time = (10 - t)
Distance = r*t = 24(10 - t)
D = D
12t = 24(10-t)
12t = 240 - 24t
Divide by 12:
t = 20 - 2t
3t = 20
t = \(\frac{20}{3}\)
How far upstream can the boat travel? rt = D
12 * \(\frac{20}{3}\) = 80
Answer D
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67% (02:00) correct 
, so i will respond on his behalf 
OKAY so i did some online research AND YES ! this a a relative speed concept you must KNOW AND MORE IMPORTANTLY - REMEMBER IT ! 
